Stability, intermittency and universal Thorpe length distribution in a laboratory turbulent stratified shear flow

DOE Office of Scientific and Technical Information (OSTI.GOV)

Odier, Philippe; Ecke, Robert E.

Stratified shear flows occur in many geophysical contexts, from oceanic overflows and river estuaries to wind-driven thermocline layers. In this study, we explore a turbulent wall-bounded shear flow of lighter miscible fluid into a quiescent fluid of higher density with a range of Richardson numbersmore » $$0.05\\lesssim Ri\\lesssim 1$$. In order to find a stability parameter that allows close comparison with linear theory and with idealized experiments and numerics, we investigate different definitions of$Ri$$. We find that a gradient Richardson number defined on fluid interface sections where there is no overturning at or adjacent to the maximum density gradient position provides an excellent stability parameter, which captures the Miles–Howard linear stability criterion. For small $$Ri$$ the flow exhibits robust Kelvin–Helmholtz instability, whereas for larger $$Ri$$ interfacial overturning is more intermittent with less frequent Kelvin–Helmholtz events and emerging Holmboe wave instability consistent with a thicker velocity layer compared with the density layer. We compute the perturbed fraction of interface as a quantitative measure of the flow intermittency, which is approximately 1 for the smallest $$Ri$$ but decreases rapidly as $$Ri$ increases, consistent with linear theory. For the perturbed regions, we use the Thorpe scale to characterize the overturning properties of these flows. The probability distribution of the non-zero Thorpe length yields a universal exponential form, suggesting that much of the overturning results from increasingly intermittent Kelvin–Helmholtz instability events. Finally, the distribution of turbulent kinetic energy, conditioned on the intermittency fraction, has a similar form, suggesting an explanation for the universal scaling collapse of the Thorpe length distribution.« less

Stability, intermittency and universal Thorpe length distribution in a laboratory turbulent stratified shear flow

DOE PAGES

Odier, Philippe; Ecke, Robert E.

2017-02-21

Stratified shear flows occur in many geophysical contexts, from oceanic overflows and river estuaries to wind-driven thermocline layers. In this study, we explore a turbulent wall-bounded shear flow of lighter miscible fluid into a quiescent fluid of higher density with a range of Richardson numbersmore » $$0.05\\lesssim Ri\\lesssim 1$$. In order to find a stability parameter that allows close comparison with linear theory and with idealized experiments and numerics, we investigate different definitions of$Ri$$. We find that a gradient Richardson number defined on fluid interface sections where there is no overturning at or adjacent to the maximum density gradient position provides an excellent stability parameter, which captures the Miles–Howard linear stability criterion. For small $$Ri$$ the flow exhibits robust Kelvin–Helmholtz instability, whereas for larger $$Ri$$ interfacial overturning is more intermittent with less frequent Kelvin–Helmholtz events and emerging Holmboe wave instability consistent with a thicker velocity layer compared with the density layer. We compute the perturbed fraction of interface as a quantitative measure of the flow intermittency, which is approximately 1 for the smallest $$Ri$$ but decreases rapidly as $$Ri$ increases, consistent with linear theory. For the perturbed regions, we use the Thorpe scale to characterize the overturning properties of these flows. The probability distribution of the non-zero Thorpe length yields a universal exponential form, suggesting that much of the overturning results from increasingly intermittent Kelvin–Helmholtz instability events. Finally, the distribution of turbulent kinetic energy, conditioned on the intermittency fraction, has a similar form, suggesting an explanation for the universal scaling collapse of the Thorpe length distribution.« less

Mantle flow tectonics - The influence of a ductile lower crust and implications for the formation of topographic uplands on Venus

NASA Technical Reports Server (NTRS)

Bindschadler, Duane L.; Parmentier, E. Marc

1990-01-01

The crust and mantle of Venus can be represented by a model of a layered structure stratified in both density and viscosity. This structure consists of a brittle-elastic upper crustal layer; a ductile weaker crustal layer; a strong upper mantle layer, about 10 percent denser than the crust; and a weaker substrate, representing the portion of the mantle in which convective flow occurs which is a primary source of large-scale topographic and tectonic features. This paper examines the interactions between these four layers and the mantle flow driven by thermal or compositional variations. Solutions are found for a flow driven by a buoyancy-force distribution within the mantle and by relief at the surface and crust-mantle boundary. It is shown that changes in crustal thickness are driven by vertical normal stresses due to mantle flow and by shear coupling of horizontal mantle flow into the crust.

Seasonal variability of Dinophysis spp. and Protoceratium reticulatum associated to lipophilic shellfish toxins in a strongly stratified Chilean fjord

NASA Astrophysics Data System (ADS)

Alves-de-Souza, Catharina; Varela, Daniel; Contreras, Cristóbal; de La Iglesia, Pablo; Fernández, Pamela; Hipp, Byron; Hernández, Cristina; Riobó, Pilar; Reguera, Beatriz; Franco, José M.; Diogène, Jorge; García, Carlos; Lagos, Néstor

2014-03-01

The fine scale vertical distribution of Dinophysis spp. and Protoceratium reticulatum (potential producers of lipophilic shellfish toxins, LSTs) and its relation with LSTs in shellfish was studied in Reloncaví fjord, a strongly stratified system in Southern Chile. Samples were taken over two years from late spring to early autumn (2007-2008 period) and from early spring to late summer (2008-2009 period). Dinophysis spp., in particular Dinophysis acuminata, were always detected, often forming thin layers in the region of the salinity driven pycnocline, with cell maxima for D. acuminata of 28.5×103 cells L-1 in March 2008 and 17.1×103 cells L-1 in November 2008. During the 2008-2009 sampling period, blooms of D. acuminata co-occurred with high densities of cryptophyceans and the ciliate Mesodinium spp. The highest levels of pectenotoxin-2 (PTX-2; 2.2 ng L-1) were found in the plankton in February 2009, associated with moderate densities of D. acuminata, Dinophysis tripos and Dinophysis subcircularis (0.1-0.6×103 cells L-1). However, only trace levels of PTX-2 were observed in bivalves at that time. Dinophysistoxin (DTX-1 and DTX-3) levels in bivalves and densities of Dinophysis spp. were not well correlated. Low DTX levels in bivalves observed during a major bloom of D. acuminata in March 2008 suggested that there is a large seasonal intraspecific variability in toxin content of Dinophysis spp. driven by changes in population structure associated with distinct LST toxin profiles in Reloncaví fjord during the study period. A heterogeneous vertical distribution was also observed for P. reticulatum, whose presence was restricted to summer months. A bloom of this species of 2.2×103 cells L-1 at 14 m depth in February 2009 was positively correlated with high concentrations of yessotoxins in bivalves (51-496 ng g-1) and plankton samples (3.2 ng L-1). Our results suggest that a review of monitoring strategies for Dinophysis spp. in strongly stratified fjord systems should be carried out. They also indicate that early warning of LST events based on Dinophysis cell numbers are not reliable for seafood control.

On the stability analysis of sharply stratified shear flows

NASA Astrophysics Data System (ADS)

Churilov, Semyon

2018-05-01

When the stability of a sharply stratified shear flow is studied, the density profile is usually taken stepwise and a weak stratification between pycnoclines is neglected. As a consequence, in the instability domain of the flow two-sided neutral curves appear such that the waves corresponding to them are neutrally stable, whereas the neighboring waves on either side of the curve are unstable, in contrast with the classical result of Miles (J Fluid Mech 16:209-227, 1963) who proved that in stratified flows unstable oscillations can be only on one side of the neutral curve. In the paper, the contradiction is resolved and changes in the flow stability pattern under transition from a model stepwise to a continuous density profile are analyzed. On this basis, a simple self-consistent algorithm is proposed for studying the stability of sharply stratified shear flows with a continuous density variation and an arbitrary monotonic velocity profile without inflection points. Because our calculations and the algorithm are both based on the method of stability analysis (Churilov J Fluid Mech 539:25-55, 2005; ibid, 617, 301-326, 2008), which differs essentially from usually used, the paper starts with a brief review of the method and results obtained with it.

Salt-Finger Convection in a Stratified Fluid Layer Induced by Thermal and Solutal Capillary Motion

NASA Technical Reports Server (NTRS)

Chen, Chuan F.; Chan, Cho Lik

1996-01-01

Salt-finger convection in a double-diffusive system is a motion driven by the release of gravitational potential due to different diffusion rates. Normally, when the gravitational field is reduced, salt-finger convection together with other convective motions driven by buoyancy forces will be rapidly suppressed. However, because the destabilizing effect of the concentration gradient is amplified by the Lewis number, with values varying from 10(exp 2) for aqueous salt solutions to 10 (exp 4) for liquid metals, salt-finger convection may be generated at much reduced gravity levels. In the microgravity environment, the surface tension gradient assumes a dominant role in causing fluid motion. In this paper, we report on some experimental results showing the generation of salt-finger convection due to capillary motio on the surface of a stratified fluid layer. A numerical simulation is presented to show the cause of salt-finger convection.

The `Henry Problem' of `density-driven' groundwater flow versus Tothian `groundwater flow systems' with variable density: A review of the influential Biscayne aquifer data.

NASA Astrophysics Data System (ADS)

Weyer, K. U.

2017-12-01

Coastal groundwater flow investigations at the Biscayne Bay, south of Miami, Florida, gave rise to the concept of density-driven flow of seawater into coastal aquifers creating a saltwater wedge. Within that wedge, convection-driven return flow of seawater and a dispersion zone were assumed by Cooper et al. (1964) to be the cause of the Biscayne aquifer `sea water wedge'. This conclusion was based on the chloride distribution within the aquifer and on an analytical model concept assuming convection flow within a confined aquifer without taking non-chemical field data into consideration. This concept was later labelled the `Henry Problem', which any numerical variable density flow program must be able to simulate to be considered acceptable. Both, `density-driven flow' and Tothian `groundwater flow systems' (with or without variable density conditions) are driven by gravitation. The difference between the two are the boundary conditions. 'Density-driven flow' occurs under hydrostatic boundary conditions while Tothian `groundwater flow systems' occur under hydrodynamic boundary conditions. Revisiting the Cooper et al. (1964) publication with its record of piezometric field data (heads) showed that the so-called sea water wedge has been caused by discharging deep saline groundwater driven by gravitational flow and not by denser sea water. Density driven flow of seawater into the aquifer was not found reflected in the head measurements for low and high tide conditions which had been taken contemporaneously with the chloride measurements. These head measurements had not been included in the flow interpretation. The very same head measurements indicated a clear dividing line between shallow local fresh groundwater flow and saline deep groundwater flow without the existence of a dispersion zone or a convection cell. The Biscayne situation emphasizes the need for any chemical interpretation of flow pattern to be supported by head data as energy indicators of flow fields. At the Biscayne site density-driven flow of seawater did and does not exist. Instead this site and the Florida coast line in general are the end points of local fresh and regional saline groundwater flow systems driven by gravity forces and not by density differences.

Convective thermal fluxes in unsteady non-homogeneous flows generating complex three dimensional vorticity patterns

NASA Astrophysics Data System (ADS)

Tellez Alvarez, Jackson David; Redondo, Jose Manuel; Sanchez, Jesu Mary

2016-04-01

The improvements in experimental methods and high resolution image analysis are nowadays able to detect subtle changes in the structure of the turbulence over a wide range of temporal and spatial scales [1], we compare the scaling shown by different mixing fronts driven by buoyancy that form convective driven mixing. We use PIV and density front tracking in several experimental configurations akin to geophysical overturning [2, 3]. We parametrize the role of unstable stratification by means of the Rayleigh and Atwood numbers and compare the scaling and the multifractal structure functions of the different markers used to visualize the non-homogeneous. Both reactive and passive scalar tracers are used to investigate the mixing structure and the intermittency of the flow. Different initial conditions are compared and the mixing efficiency of the overall turbulent process is evaluated [4 - 6]. Diffusion is measured in the transition from a homogeneous linearly stratified fluid to a cellular or layered structure by means of Thermoelectric generated heating and cooling [2, 4]. Patterns arise by setting up a convective flow generated by a buoyant heat flux either in the base or in a side wall of the convective enclosure [1, 6]. The experiments described here investigate high Prandtl number mixing using brine or sugar solutions and fresh water in order to form a density interface and low Prandtl number mixing with only temperature gradients [7]. The set of dimensionless parameters define conditions of numeric and small scale laboratory modeling of environmental flows. Fields of velocity, density and their gradients were computed and visualized [8, 9]. When convective heating and cooling takes place the combination of internal waves and buoyant turbulence is much more complicated if the Rayleigh and Reynolds numbers are high in order to study entrainment and mixing. The experiments described here investigate high Prandtl number mixing using salt or sugar solutions and fresh water in order to form density interfaces. The Reynolds number can be reduced adding Glicerine the set of dimensionless parameters define different conditions of both numeric and small scale laboratory applied often in modeling environmental flows. Fields of velocity, density and their gradients are computed using advanced visualization [8 9]. Visualizations are performed by PIV, Particle tracking and shadowgraph. When convective heating and cooling takes place the patterns depend on the parameter space region of the initial conditions We also map the different transitions between two and three dimensional convection in an enclosure with several complex driven flows. The size of the water tank is of 0.2 x 0.2 x 0.1 m and the heat sources or sinks can be regulated both in power and sign [2-4]. The thermal convective driven flows are generated by Seebeck and Peltier effects in 4 wall extended positions of 0.05 x 0.05 cm each. The parameter range of convective cell array varies strongly with the Topology of the boundary conditions. At present side heat fluxes are considered and estimated as a function of Rayleigh, Peclet and Nusselt numbers, [4-6] The evolution of the mixing fronts are compared and the topological characteristics of the merging of plumes and jets in different configurations presenting detailed comparison of the evolution of RM and RT, Jets and Plumes in overall mixing. The relation between structure functions, fractal analysis and spectral analysis can be very useful to determine the evolution of scales. Experimental and numerical results on the advance of a mixing or non-mixing front occurring at a density interface due to body forces [12] can be compared with the convective fronts. The evolution of the turbulent mixing layer and its complex configuration is studied taking into account the dependence on the initial modes at the early stages, Self-similar information [13]. Spectral and Fractal analysis on the images seems very useful in order to estimate dominant mixing structures as well as the basic instabilities than drive the turbulent direct and inverse cascades [12]. References [1] Mahjoub O.B.; Redondo J.M.; and Babiano A. (2000). Hyerarchy flux in nonhomogeneous flows in turbulent diffusion in the environment Eds. Redondo J.M. and Babiano A. 249-260. [2] Redondo J.M. (1992). Termodinámica de los procesos irreversibles, efectos termoeléctricos. Rev. Termoelectricidad2, 16-29. AIT. Pamplona. [3] Dalziel, S. B. (1994). Perturbations and coherent flow in Rayleigh-Taylor instability in 4th International Workshop on the Physics of Compressible Turbulent Mixing, ed. P. F. Linden, D. L. Youngs, & S. B. Dalziel; 32-41. [4] Redondo J.M.; Sanchez J.M.; Pascual I.; Noriega, G.F. (1995). Thermoelectric regulation for electric cabinets: XVI International Conference in Thermoelectrics, 02-G20. Ed. V. Vedernikov. 456-468. St. Petersburg, Russia. [5] Mahjoub, O.B; Redondo J.M.; Babiano A. (1998).Structure functions in complex flows: Applied Scientific Research. 59, 299-313. Kluwer. [6] Matulka A.; Redondo J. M.; and Carrillo A. (2008). Experiments in stratified and rotating decaying 2D flows, Il Nuovo Cimento 31, 5-6, 757-770. 2008. [7] Redondo, J.M.; Tellez, J; Sotillos, L.; Gonzalez-Nieto, Pilar L.; Sanchez, J.M.; Furmanek P.,; Diez M (2014). Complex Convective Thermal Fluxes and Vorticity Structure: Geophysical Research Abstracts Vol. 17, EGU2015-14773, 2015 - EGU General Assembly 2015. [8] Nicolleau, F.C.G.A.; Cambon, C.; Redondo, J.M.; Vassilicos, J.C.; Reeks, M.; Nowakowski, A.F. (Eds.) (2012). New Approaches in Modeling Multiphase Flows and Dispersion in Turbulence, Fractal Methods and Synthetic Turbulence: ERCOFTAC Series. [9] Redondo J. M.; Sanchez M. A.; and Cantalapiedra I. R. (1995). Turbulent Mechanisms in Stratified Flows, Dynamics of Atmospheres and Oceans, 23, 454-462. [10] Matulka A.; Redondo J.M.; and Carrillo A. (2008) Experiments in stratified and rotating decaying 2D flows. II Nuovo Cimento 31, 5-6, 757-770. 2008. [11] Fraunie P., Berreba S. Chashechkin Yu.D., Velasco D. and Redondo J.M. (2008). Large eddy simulation and laboratory experiments on the decay of grid wakes in strongly stratified flows. II Nuovo Cimento C 31, 909-930. [12] Matulka, A.; López, P.; Redondo, J. M.; and Tarquis, A. (2014). On the entrainment coefficient in a forced plume: quantitative effects of source parameters. Nonlin. Processes Geophys. 21, 269-278. [13] Castilla R.; Oñate E.; and Redondo J.M. (2007) Models, Experiments and Computations in Turbulence. CIMNE, Barcelona, 255.

Characterizing the performance of an affordable, multichannel conductivity probe for density measurements in stratified flows

NASA Astrophysics Data System (ADS)

Subramanian, Balaji; Carminati, Marco; Luzzatto-Fegiz, Paolo

2017-11-01

In stratified flows, conductivity (combined with temperature) is often used to measure density. The conductivity probes typically used can resolve very fine spatial scales, but on the downside they are fragile, expensive, sensitive to environmental noise and have only single channel capability. Recently a low-cost, robust, arduino-based probe called Conduino was developed, which can be valuable in a wide range of applications where resolving extremely small spatial scales is not needed. This probe uses micro-USB connectors as actual conductivity sensors with a custom designed electronic board for simultaneous acquisition from multiple probes, with conductivity resolution comparable to commercially available PME conductivity probe. A detailed assessment of performance of this Conduino probe is described here. To establish time response and sensitivity as a function of electrode geometry, we build a variety of shapes for different kinds of applications, with tip spacing ranging from 0.5-2.5 mm, and with electrode length ranging from 2.3-6 mm. We set up a two-layer density profile and traverse it rapidly, yielding a time response comparable to PME. The Conduino's multi-channel capability is used to operate probe arrays, which helps to construct density fields in stratified flows.

Electromagnetic Properties Analysis on Hybrid-driven System of Electromagnetic Motor

NASA Astrophysics Data System (ADS)

Zhao, Jingbo; Han, Bingyuan; Bei, Shaoyi

2018-01-01

The hybrid-driven system made of permanent-and electromagnets applied in the electromagnetic motor was analyzed, equivalent magnetic circuit was used to establish the mathematical models of hybrid-driven system, based on the models of hybrid-driven system, the air gap flux, air-gap magnetic flux density, electromagnetic force was proposed. Taking the air-gap magnetic flux density and electromagnetic force as main research object, the hybrid-driven system was researched. Electromagnetic properties of hybrid-driven system with different working current modes is studied preliminary. The results shown that analysis based on hybrid-driven system can improve the air-gap magnetic flux density and electromagnetic force more effectively and can also guarantee the output stability, the effectiveness and feasibility of the hybrid-driven system are verified, which proved theoretical basis for the design of hybrid-driven system.

Undersized description on motile gyrotactic micro-organisms individualities in MHD stratified water-based Newtonian nanofluid

NASA Astrophysics Data System (ADS)

Rehman, Khalil Ur; Malik, Aneeqa Ashfaq; Tahir, M.; Malik, M. Y.

2018-03-01

The current pagination summarized the influence of bio-convection Schmidt number, bio-convection Peclet number and micro-organisms concentration difference parameter on the density of motile gyrotactic micro-organisms when they have interaction with the thermally stratified magneto-nanofluid flow past a vertical stretching surface. It is observed that the density of motile microorganisms is the decreasing function of the bio-convection Schmidt and Peclet numbers. It is trusted that the outcomes of present analysis will serve as a helping source for the upcoming developments regarding individualities of motile gyrotactic micro-organisms subject to boundary layer flows induced by stretching surfaces.

A frozen record of density-driven crustal overturn in lava lakes: The example of Kilauea Iki 1959

USGS Publications Warehouse

Stovall, W.K.; Houghton, Bruce F.; Harris, A.J.L.; Swanson, D.A.

2009-01-01

Lava lakes are found at basaltic volcanoes on Earth and other planetary bodies. Density-driven crustal foundering leading to surface renewal occurs repeatedly throughout the life of a lava lake. This process has been observed and described in a qualitative sense, but due to dangerous conditions, no data has been acquired to evaluate the densities of the units involved. Kilauea Iki pit crater in Hawai'i houses a lava lake erupted during a 2 month period in 1959. Part of the surface of the Kilauea Iki lake now preserves the frozen record of a final, incomplete, crustal-overturn cycle. We mapped this region and sampled portions of the foundering crust, as well as overriding and underlying lava, to constrain the density of the units involved in the overturn process. Overturn is driven by the advance of a flow front of fresh, low-density lava over an older, higher density surface crust. The advance of the front causes the older crust to break up, founder, and dive downwards into the lake to expose new, hot, low-density lava. We find density differences of 200 to 740 kg/m3 between the foundering crust and over-riding and under-lying lava respectively. In this case, crustal overturn is driven by large density differences between the foundering and resurfacing units. These differences lead, inevitably, to frequent crustal renewal: simple density differences between the surface crust and underlying lake lava make the upper layers of the lake highly unstable. ?? Springer-Verlag 2008.

Turbulence suppression at water density interfaces: observations under moderate wind forcing.

NASA Astrophysics Data System (ADS)

Marcello Falcieri, Francesco; Kanth, Lakshmi H.; Benetazzo, Alvise; Bergamasco, Andrea; Bonaldo, Davide; Barbariol, Francesco; Malačič, Vlado; Sclavo, Mauro; Carniel, Sandro

2016-04-01

Water column stratification has a strong influence on the behaviour of turbulence kinetic energy (TKE) dissipation rates. Density gradient interfaces, due to thermohaline characteristics and to suspended sediment concentration, can act as a barrier and significantly damp TKE. Between January 30th - February 4th 2014 (CARPET2014 oceanographic campaign on R/V URANIA) we collected the very first turbulence data in the Gulf of Trieste (a small bay located in the North-eartern part of the Adriatic Sea). Observation consisted of 38 CTD casts and 478 microstructure profiles (145 ensembles) collected with a free-falling probe (MSS90L). Among those 48 were grouped in three sets of yoyo casts, each lasting for about 12 consecutive hours. The meteorological conditions during the campaign were of moderate wind (average wind speed 10 m s-1) and heat flux (net negative heat flux ranging from 150 to 400 W m-2). The water column characteristics in the Gulf during the campaign evolved from well-mixed to stratified conditions with waters intruding from the Adriatic Sea at the bottom. Two types of water intrusions were found during yoyo casts: one coming from the Adriatic Sea northern coast (i.e. warmer, saltier and more turbid) and one coming from the open sea in front of the Po Delta (i.e. cooler, fresher and less turbid). Our observations show that under moderate wind forcing, the GOT was not completely mixed due to the interfaces created by the bottom waters intruding from the open sea. The comparison of microstructure profiles collected during well mixed and stratified conditions permitted us to highlight the effect of different stratification on TKE dissipation rates. While during well mixed condition TKE profiles are governed just by their forcing, the two intrusions showed different impacts on TKE dissipation rate profiles. The coastal one, with high turbidity, acted as a barrier to surface driven turbulence dumping it of almost two order of magnitude, while the one coming from the open sea, with low sediment concentrations and a smaller vertical density gradient, was not able to suppress downward penetration of turbulence from the surface.

Background oriented schlieren in a density stratified fluid.

PubMed

Verso, Lilly; Liberzon, Alex

2015-10-01

Non-intrusive quantitative fluid density measurement methods are essential in the stratified flow experiments. Digital imaging leads to synthetic schlieren methods in which the variations of the index of refraction are reconstructed computationally. In this study, an extension to one of these methods, called background oriented schlieren, is proposed. The extension enables an accurate reconstruction of the density field in stratified liquid experiments. Typically, the experiments are performed by the light source, background pattern, and the camera positioned on the opposite sides of a transparent vessel. The multimedia imaging through air-glass-water-glass-air leads to an additional aberration that destroys the reconstruction. A two-step calibration and image remapping transform are the key components that correct the images through the stratified media and provide a non-intrusive full-field density measurements of transparent liquids.

Vertical Transport of Sediment from Muddy Buoyant River Plumes in the Presence of Different Modes of Interfacial Instabilities

NASA Astrophysics Data System (ADS)

Strom, K.; Rouhnia, M.

2016-12-01

Previous studies have suggested that sedimentation from buoyant, muddy plumes lofting over clear saltwater can take place at rates higher than that expected from individual particle settling (i.e., CWs). Two potential drivers of enhanced sedimentation are flocculation and interfacial instabilities. We experimentally measured the sediment fluxes from each of these processes using two sets of laboratory experiments that investigate two different modes of instability, one driven by sediment settling and one driven by fluid shear. The settling-driven and shear-driven instability experiments were carried out in a stagnant stratification tank and a stratification flume respectively. In both sets, continuous interface monitoring and concentration measurements were made to observe developments of instabilities and their effects on the removal of sediment. Floc size was measured during the experiments using a floc camera and image analysis routines. This presentation will provide an overview of the stagnant tank experiments, but will focus on results from the stratified flume experiments and an analysis that attempts to synthesizes the results from the entirety of the study. The results from the stratified flume experiments show that under shear instabilities, the effective settling velocity is greater than the floc settling velocity, and that the rate increases with plume velocity and interface mixing. The difference between effective and floc settling velocity was denoted as the shear-induced settling velocity. This rate was found to be a strong function of the Richardson number, and was attributed to mixing processes at the interface. Conceptual and empirical analysis shows that the shear-induced settling velocity is proportional to URi-2. The resulting effective settling velocity models developed from these experiments are then used to examine the rates and potential locations of operations of these mechanism over the length of a river mouth plume.

On the propagation of particulate gravity currents in circular and semi-circular channels partially filled with homogeneous or stratified ambient fluid

NASA Astrophysics Data System (ADS)

Zemach, T.; Chiapponi, L.; Petrolo, D.; Ungarish, M.; Longo, S.; Di Federico, V.

2017-10-01

We present a combined theoretical-experimental investigation of particle-driven gravity currents advancing in circular cross section channels in the high-Reynolds number Boussinesq regime; the ambient fluid is either homogeneous or linearly stratified. The predictions of the theoretical model are compared with experiments performed in lock-release configuration; experiments were performed with conditions of both full-depth and partial-depth locks. Two different particles were used for the turbidity current, and the full range 0 ≤S ≤1 of the stratification parameter was explored (S = 0 corresponds to the homogeneous case and S = 1 when the density of the ambient fluid and of the current are equal at the bottom). In addition, a few saline gravity currents were tested for comparison. The results show good agreement for the full-depth configuration, with the initial depth of the current in the lock being equal to the depth of the ambient fluid. The agreement is less good for the partial-depth cases and is improved by the introduction of a simple adjustment coefficient for the Froude number at the front of the current and accounting for dissipation. The general parameter dependencies and behaviour of the current, although influenced by many factors (e.g., mixing and internal waves), are well predicted by the relatively simple model.

Sun-stirred Kraken Mare: Circulation in Titan's seas induced by solar heating and methane precipitation

NASA Astrophysics Data System (ADS)

Tokano, T.; Lorenz, R. D.

2015-10-01

Density-driven circulation in Titan's seas forced by solar heating and methane evaporation/precipitation is simulated by an ocean circulation model. If the sea is transparent to sunlight, solar heating can induce anti-clockwise gyres near the sea surface and clockwise gyres near the sea bottom. The gyres are in geostrophic balance between the radially symmetric pressure gradient force and Coriolis force. If instead the sea is turbid and most sunlight is absorbed near the sea surface, the sea gets stratified in warm seasons and the circulation remains weak. Strong summer precipitation at high latitudes causes compositional stratification and increase of the nearsurface methane mole fraction towards the north pole. The resultant latitudinal density contrast drives a meridional overturning with equatorward currents near the sea surface and poleward currents near the sea bottom. Weak precipitation induces gyres rather than meridional overturning.

Progress toward Kelvin-Helmholtz instabilities in a High-Energy-Density Plasma on the Nike laser

NASA Astrophysics Data System (ADS)

Harding, E. C.; Drake, R. P.; Gillespie, R. S.; Grosskopf, M. J.; Huntington, C. M.; Aglitskiy, Y.; Weaver, J. L.; Velikovich, A. L.; Plewa, T.; Dwarkadas, V. V.

2008-04-01

In the realm of high-energy-density (HED) plasmas, there exist three primary hydrodynamic instabilities of concern: Rayleigh-Taylor (RT), Richtmyer-Meshkov (RM), and Kelvin-Helmholtz (KH). Although the RT and the RM instabilities have been readily observed and diagnosed in the laboratory, the KH instability remains relatively unexplored in HED plasmas. Unlike the RT and RM instabilities, the KH instability is driven by a lifting force generated by a strong velocity gradient in a stratified fluid. Understanding the KH instability mechanism in HED plasmas will provide essential insight into oblique shock systems, jets, mass stripping, and detailed RT-spike development. In addition, our KH experiment will help provide the groundwork for future transition to turbulence experiments. We present 2D FLASH simulations and experimental data from our initial attempts to create a pure KH system using the Nike laser at the Naval Research Laboratory.

Canopy reflectance modeling in a tropical wooded grassland

NASA Technical Reports Server (NTRS)

Simonett, David

1988-01-01

The Li-Strahler canopy reflectance model, driven by LANDSAT Thematic Mapper (TM) data, provided regional estimates of tree size and density in two bioclimatic zones in Africa. This model exploits tree geometry in an inversion technique to predict average tree size and density from reflectance data using a few simple patameters measured in the field and in the imagery. Reflectance properties of the trees were measured in the study sites using a pole-mounted radiometer. The measurements showed that the assumptions of the simple Li-Strahler model are reasonable for these woodlands. The field radiometer measurements were used to calculate the normalized difference vegetation index (NDVI), and the integrated NDVI over the canopy was related to crown volume. Predictions of tree size and density from the canopy model were used with allometric equations from the literature to estimate woody biomass and potential foliar biomass for the sites and for the regions. Estimates were compared with independent measurements made in the Sahelian sites, and to typical values from the literature for these regions and for similar woodlands. In order to apply the inversion procedure regionally, an area must first be stratified into woodland cover classes, and dry-season TM data were used to generate a stratum map of the study areas with reasonable accuracy. The method used was unsupervised classification of multi-data principal components images.

Observations of a tidal intrusion front in a tidal channel

NASA Astrophysics Data System (ADS)

Lu, Shasha; Xia, Xiaoming; Thompson, Charlie E. L.; Cao, Zhenyi; Liu, Yifei

2017-11-01

A visible front indicated by a surface colour change, and sometimes associated with foam or debris lines, was observed in a tidal channel during neap tide. This is an example of a tidal intrusion front occurring in the absence of sudden topographical changes or reversing flows, typically reported to be associated with such fronts. Detailed Acoustic Doppler Current Profiler and conductivity/temperature/depth measurements were taken on repeated transects both with fronts apparent and with fronts absent. The results indicated that the front occurred as a result of stratification, which was sustained by the buoyancy flux and the weak tide-induced mixing during neap ebb tide. The stronger tide-induced mixing during spring tide restrained stratification, leading to the absence of a front. The mechanism of the frontogenesis was the density gradient between the stratified water formed during neap ebb tide, and the more mixed seawater during neap flood tide; thus, the water on the landward (southwestern) side of the front was stratified, and that on the seaward side (northeastern) of the front was vertically well mixed. Gradient Richardson number estimates suggest that the flow between the stratified and mixed water was near the threshold 0.25 for shear instability. Meanwhile, the density gradient would provide an initial baroclinic contribution to velocity convergence, which is indicated by the accumulation of buoyant matter such as foam, grass, and debris into a sharply defined line along the surface. The front migrates with the flood current, with a local maximum towards the eastern side of the channel, leading to an asymmetrical shape with the eastern side of the front driven further into the Tiaozhoumen tidal channel.

Effects of Density Stratification in Compressible Polytropic Convection

NASA Astrophysics Data System (ADS)

Manduca, Cathryn M.; Anders, Evan H.; Bordwell, Baylee; Brown, Benjamin P.; Burns, Keaton J.; Lecoanet, Daniel; Oishi, Jeffrey S.; Vasil, Geoffrey M.

2017-11-01

We study compressible convection in polytropically-stratified atmospheres, exploring the effect of varying the total density stratification. Using the Dedalus pseudospectral framework, we perform 2D and 3D simulations. In these experiments we vary the number of density scale heights, studying atmospheres with little stratification (1 density scale height) and significant stratification (5 density scale heights). We vary the level of convective driving (quantified by the Rayleigh number), and study flows at similar Mach numbers by fixing the initial superadiabaticity. We explore the differences between 2D and 3D simulations, and in particular study the equilibration between different reservoirs of energy (kinetic, potential and internal) in the evolved states.

Numerical modeling of a sub Plinian eruption at La Soufrière de Guadeloupe: implications for pyroclastic density currents hazard assessment.

NASA Astrophysics Data System (ADS)

Esposti Ongaro, Tomaso; Neri, Augusto; Komorowski, Jean-Christophe

2013-04-01

We present three-dimensional numerical simulations of a sub-Plinian eruptive scenario at La Soufrière de Guadeloupe, aimed at assessing the capability of pyroclastic density currents to reach the inhabited regions on the volcano slopes, in case of the future resumption of the explosive activity. The selected eruptive scenario is similar to that hypothesized for the 1530 a.D. eruption, but several eruptive conditions have been analyzed to account for different behaviours of the eruptive column and percentages of collapse. Numerical results describe, in 3D and in time, the formation, instability and partial collapse of the eruptive column, and the simultaneous formation of a convective plume and several branched pyroclastic density currents. The proximal volcano morphology, characterized by the presence of ancient caldera rims and the remnants of the old edifice, controls the areal distribution of the collapsed material and the paths of channelized flows along the incised topography. The analysis of the 3D runs suggests that partial collapse scenarios produce steeply stratified pyroclastic density currents, which are strongly controlled by the topography and whose propagation is likely driven by the dynamics of the dense, basal layer. Although vertical grid size still does not allow the resolution of the dynamics of such concentrated flows, preliminary georeferenced maps of pyroclastic density currents' hazardous actions (temperature and dynamic pressure) provide interesting and useful information which can serve as a basis for elaborating a quantitative framework for the assessment of their impact on vulnerable infrastructures, networks, and population.

Slow Magnetosonic Waves and Fast Flows in Active Region Loops

NASA Technical Reports Server (NTRS)

Ofman, L.; Wang, T. J.; Davila, J. M.

2012-01-01

Recent extreme ultraviolet spectroscopic observations indicate that slow magnetosonic waves are present in active region (AR) loops. Some of the spectral data were also interpreted as evidence of fast (approx 100-300 km/s) quasiperiodic flows. We have performed three-dimensional magnetohydrodynamic (3D MHD) modeling of a bipolar AR that contains impulsively generated waves and flows in coronal loops. The model AR is initiated with a dipole magnetic field and gravitationally stratified density, with an upflow-driven steadily or periodically in localized regions at the footpoints of magnetic loops. The resulting flows along the magnetic field lines of the AR produce higher density loops compared to the surrounding plasma by injection of material into the flux tubes and the establishment of siphon flow.We find that the impulsive onset of flows with subsonic speeds result in the excitation of damped slow magnetosonic waves that propagate along the loops and coupled nonlinearly driven fast-mode waves. The phase speed of the slow magnetosonic waves is close to the coronal sound speed. When the amplitude of the driving pulses is increased we find that slow shock-like wave trains are produced. When the upflows are driven periodically, undamped oscillations are produced with periods determined by the periodicity of the upflows. Based on the results of the 3D MHD model we suggest that the observed slow magnetosonic waves and persistent upflows may be produced by the same impulsive events at the bases of ARs.

Particle Image Velocimetry Study of Density Current Fronts

ERIC Educational Resources Information Center

Martin, Juan Ezequiel

2009-01-01

Gravity currents are flows that occur when a horizontal density difference causes fluid to move under the action of gravity; density currents are a particular case, for which the scalar causing the density difference is conserved. Flows with a strong effect of the horizontal density difference, even if only partially driven by it--such as the…

TURBULENCE AND STEADY FLOWS IN THREE-DIMENSIONAL GLOBAL STRATIFIED MAGNETOHYDRODYNAMIC SIMULATIONS OF ACCRETION DISKS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Flock, M.; Dzyurkevich, N.; Klahr, H.

2011-07-10

We present full 2{pi} global three-dimensional stratified magnetohydrodynamic (MHD) simulations of accretion disks. We interpret our results in the context of protoplanetary disks. We investigate the turbulence driven by the magnetorotational instability (MRI) using the PLUTO Godunov code in spherical coordinates with the accurate and robust HLLD Riemann solver. We follow the turbulence for more than 1500 orbits at the innermost radius of the domain to measure the overall strength of turbulent motions and the detailed accretion flow pattern. We find that regions within two scale heights of the midplane have a turbulent Mach number of about 0.1 and amore » magnetic pressure two to three orders of magnitude less than the gas pressure, while in those outside three scale heights the magnetic pressure equals or exceeds the gas pressure and the turbulence is transonic, leading to large density fluctuations. The strongest large-scale density disturbances are spiral density waves, and the strongest of these waves has m = 5. No clear meridional circulation appears in the calculations because fluctuating radial pressure gradients lead to changes in the orbital frequency, comparable in importance to the stress gradients that drive the meridional flows in viscous models. The net mass flow rate is well reproduced by a viscous model using the mean stress distribution taken from the MHD calculation. The strength of the mean turbulent magnetic field is inversely proportional to the radius, so the fields are approximately force-free on the largest scales. Consequently, the accretion stress falls off as the inverse square of the radius.« less

Turbulent Heat Transfer from a Thermally Forced Boundary in a Stratified Fluid

NASA Astrophysics Data System (ADS)

Burns, K. J.; Wells, A.; Flierl, G.

2017-12-01

When a marine-terminating glacier melts into a stratified ocean, a buoyancy-driven flow develops along the ice surface. The resulting turbulent heat and salt fluxes provide a key feedback on the ice melting rate. To build insight into such flows, we consider direct numerical simulations of an analogue problem with convection driven by a thermally forced sidewall in a stably stratified Boussinesq fluid. Our model considers vertical and inclined periodic channels in 2D with a constant background buoyancy gradient. When the lateral or upper boundary is given a sufficient thermal perturbation relative to the ambient, a confined and homogeneous turbulent plume emerges along the heated wall. We present a scaling analysis for the resulting heat transport across the plume, and compare it to simulations over a range of Rayleigh numbers, Prandtl numbers, and wall-inclination angles.

Extended Subadiabatic Layer in Simulations of Overshooting Convection

DOE Office of Scientific and Technical Information (OSTI.GOV)

Käpylä, Petri J.; Arlt, Rainer; Rheinhardt, Matthias

2017-08-20

We present numerical simulations of hydrodynamic overshooting convection in local Cartesian domains. We find that a substantial fraction of the lower part of the convection zone (CZ) is stably stratified according to the Schwarzschild criterion while the enthalpy flux is outward directed. This occurs when the heat conduction profile at the bottom of the CZ is smoothly varying, based either on a Kramers-like opacity prescription as a function of temperature and density or a static profile of a similar shape. We show that the subadiabatic layer arises due to nonlocal energy transport by buoyantly driven downflows in the upper partsmore » of the CZ. Analysis of the force balance of the upflows and downflows confirms that convection is driven by cooling at the surface. We find that the commonly used prescription for the convective enthalpy flux being proportional to the negative entropy gradient does not hold in the stably stratified layers where the flux is positive. We demonstrate the existence of a non-gradient contribution to the enthalpy flux, which is estimated to be important throughout the convective layer. A quantitative analysis of downflows indicates a transition from a tree-like structure where smaller downdrafts merge into larger ones in the upper parts to a structure in the deeper parts where a height-independent number of strong downdrafts persist. This change of flow topology occurs when a substantial subadiabatic layer is present in the lower part of the CZ.« less

Transition to turbulence in stratified shear flow: experiments in an inclined square duct

NASA Astrophysics Data System (ADS)

Meyer, Colin; Linden, Paul

2013-11-01

We describe laboratory experiments of countercurrent stratified shear flow in an inclined square duct. To achieve this, a long water tank was partitioned into regions of higher and lower density saltwater that are connected by an inclined square duct. The flow regime was characterized to be turbulent, intermittent, Holmboe or laminar as a function of the duct inclination, θ, and the density difference, Δρ , between the two reservoirs. The density difference and duct angle were systematically varied and a phase plane of flow regime was developed. The transition between the interrmittent regime and turbulence was experimentally determined to occur at θΔρ ~= 20 [degrees kg m-3]. This critical combination of parameters fits into the buoyancy-compensated Reynolds number scaling proposed by Brethouwer et al. (J. Fluid Mech., 2007). The turbulent interfacial thickness was found to be a function of the inclination angle, which can be predicted using the buoyancy lengthscale from Waite and Bartello (J. Fluid Mech., 2004) and others. Furthermore, we measured the density profiles at multiple points along the duct, and using these profiles, we modeled the entrainment at the interface. Support provided by the Winston Churchill Foundation of the United States.

Data-driven asthma endotypes defined from blood biomarker and gene expression data

EPA Science Inventory

The diagnosis and treatment of childhood asthma is complicated by its mechanistically distinct subtypes (endotypes) driven by genetic susceptibility and modulating environmental factors. Clinical biomarkers and blood gene expression were collected from a stratified, cross-section...

Subtidal circulation patterns in a shallow, highly stratified estuary: Mobile Bay, Alabama

USGS Publications Warehouse

Noble, M.A.; Schroeder, W.W.; Wiseman, W.J.; Ryan, H.F.; Gelfenbaum, G.

1996-01-01

Mobile Bay is a wide (25-50 km), shallow (3 m), highly stratified estuary on the Gulf coast of the United States. In May 1991 a series of instruments that measure near-surface and near-bed current, temperature, salinity, and middepth pressure were deployed for a year-long study of the bay. A full set of measurements were obtained at one site in the lower bay; all but current measurements were obtained at a midbay site. These observations show that the subtidal currents in the lower bay are highly sheared, despite the shallow depth of the estuary. The sheared flow patterns are partly caused by differential forcing from wind stress and river discharge. Two wind-driven flow patterns actually exist in lower Mobile Bay. A barotropic response develops when the difference between near-surface and near-bottom salinity is less than 5 parts per thousand. For stronger salinity gradients the wind-driven currents are larger and the response resembles a baroclinic flow pattern. Currents driven by river flows are sheared and also have a nonlinear response pattern. Only near-surface currents are driven seaward by discharges below 3000 m3/s. At higher discharge rates, surface current variability uncouples from the river flow and the increased discharge rates drive near-bed current seaward. This change in the river-forced flow pattern may be associated with a hydraulic jump in the mouth of the estuary. Copyright 1996 by the American Geophysical Union.

Visualization and simulation of density driven convection in porous media using magnetic resonance imaging

NASA Astrophysics Data System (ADS)

Montague, James A.; Pinder, George F.; Gonyea, Jay V.; Hipko, Scott; Watts, Richard

2018-05-01

Magnetic resonance imaging is used to observe solute transport in a 40 cm long, 26 cm diameter sand column that contained a central core of low permeability silica surrounded by higher permeability well-sorted sand. Low concentrations (2.9 g/L) of Magnevist, a gadolinium based contrast agent, produce density driven convection within the column when it starts in an unstable state. The unstable state, for this experiment, exists when higher density contrast agent is present above the lower density water. We implement a numerical model in OpenFOAM to reproduce the observed fluid flow and transport from a density difference of 0.3%. The experimental results demonstrate the usefulness of magnetic resonance imaging in observing three-dimensional gravity-driven convective-dispersive transport behaviors in medium scale experiments.

Topographic and sex-related differences in sleep spindles in major depressive disorder: a high-density EEG investigation.

PubMed

Plante, D T; Goldstein, M R; Landsness, E C; Peterson, M J; Riedner, B A; Ferrarelli, F; Wanger, T; Guokas, J J; Tononi, G; Benca, R M

2013-03-20

Sleep spindles are believed to mediate several sleep-related functions including maintaining disconnection from the external environment during sleep, cortical development, and sleep-dependent memory consolidation. Prior studies that have examined sleep spindles in major depressive disorder (MDD) have not demonstrated consistent differences relative to control subjects, which may be due to sex-related variation and limited spatial resolution of spindle detection. Thus, this study sought to characterize sleep spindles in MDD using high-density electroencephalography (hdEEG) to examine the topography of sleep spindles across the cortex in MDD, as well as sex-related variation in spindle topography in the disorder. All-night hdEEG recordings were collected in 30 unipolar MDD participants (19 women) and 30 age and sex-matched controls. Topography of sleep spindle density, amplitude, duration, and integrated spindle activity (ISA) were assessed to determine group differences. Spindle parameters were compared between MDD and controls, including analysis stratified by sex. As a group, MDD subjects demonstrated significant increases in frontal and parietal spindle density and ISA compared to controls. When stratified by sex, MDD women demonstrated increases in frontal and parietal spindle density, amplitude, duration, and ISA; whereas MDD men demonstrated either no differences or decreases in spindle parameters. Given the number of male subjects, this study may be underpowered to detect differences in spindle parameters in male MDD participants. This study demonstrates topographic and sex-related differences in sleep spindles in MDD. Further research is warranted to investigate the role of sleep spindles and sex in the pathophysiology of MDD. Copyright © 2012 Elsevier B.V. All rights reserved.

Non-Axisymmetric Line Driven Disc Winds II - Full Velocity Gradient

NASA Astrophysics Data System (ADS)

Dyda, Sergei; Proga, Daniel

2018-05-01

We study non-axisymetric features of 3D line driven winds in the Sobolev approximation, where the optical depth is calculated using the full velocity gradient. We find that non-axisymmetric density features, so called clumps, form primarily at the base of the wind on super-Sobolev length scales. The density of clumps differs by a factor of ˜3 from the azimuthal average, the magnitude of their velocity dispersion is comparable to the flow velocity and they produce ˜20% variations in the column density. Clumps may be observable because differences in density produce enhancements in emission and absorption profiles or through their velocity dispersion which enhances line broadening.

Probing plasma wakefields using electron bunches generated from a laser wakefield accelerator

NASA Astrophysics Data System (ADS)

Zhang, C. J.; Wan, Y.; Guo, B.; Hua, J. F.; Pai, C.-H.; Li, F.; Zhang, J.; Ma, Y.; Wu, Y. P.; Xu, X. L.; Mori, W. B.; Chu, H.-H.; Wang, J.; Lu, W.; Joshi, C.

2018-04-01

We show experimental results of probing the electric field structure of plasma wakes by using femtosecond relativistic electron bunches generated from a laser wakefield accelerator. Snapshots of laser-driven linear wakes in plasmas with different densities and density gradients are captured. The spatiotemporal evolution of the wake in a plasma density up-ramp is recorded. Two parallel wakes driven by a laser with a main spot and sidelobes are identified in the experiment and reproduced in simulations. The capability of this new method for capturing the electron- and positron-driven wakes is also shown via 3D particle-in-cell simulations.

Quantum image pseudocolor coding based on the density-stratified method

NASA Astrophysics Data System (ADS)

Jiang, Nan; Wu, Wenya; Wang, Luo; Zhao, Na

2015-05-01

Pseudocolor processing is a branch of image enhancement. It dyes grayscale images to color images to make the images more beautiful or to highlight some parts on the images. This paper proposes a quantum image pseudocolor coding scheme based on the density-stratified method which defines a colormap and changes the density value from gray to color parallel according to the colormap. Firstly, two data structures: quantum image GQIR and quantum colormap QCR are reviewed or proposed. Then, the quantum density-stratified algorithm is presented. Based on them, the quantum realization in the form of circuits is given. The main advantages of the quantum version for pseudocolor processing over the classical approach are that it needs less memory and can speed up the computation. Two kinds of examples help us to describe the scheme further. Finally, the future work are analyzed.

Stratified Magnetically Driven Accretion-Disk Winds and Their Relations To Jets

NASA Technical Reports Server (NTRS)

Fukumura, Keigo; Tombesi, Francesco; Kazanas, Demosthenes; Shrader, Chris; Behar, Ehud; Contopoulos, Ioannis

2013-01-01

We explore the poloidal structure of two-dimensional magnetohydrodynamic (MHD) winds in relation to their potential association with the X-ray warm absorbers (WAs) and the highly ionized ultra-fast outflows (UFOs) in active galactic nuclei (AGNs), in a single unifying approach. We present the density n(r, theta), ionization parameter xi(r, theta), and velocity structure v(r, theta) of such ionized winds for typical values of their fluid-to-magnetic flux ratio, F, and specific angular momentum, H, for which wind solutions become super-Alfvenic. We explore the geometrical shape of winds for different values of these parameters and delineate the values that produce the widest and narrowest opening angles of these winds, quantities necessary in the determination of the statistics of AGN obscuration. We find that winds with smaller H show a poloidal geometry of narrower opening angles with their Alfv´en surface at lower inclination angles and therefore they produce the highest line of sight (LoS) velocities for observers at higher latitudes with the respect to the disk plane. We further note a physical and spatial correlation between the X-ray WAs and UFOs that form along the same LoS to the observer but at different radii, r, and distinct values of n, xi, and v consistent with the latest spectroscopic data of radio-quiet Seyfert galaxies. We also show that, at least in the case of 3C 111, the winds' pressure is sufficient to contain the relativistic plasma responsible for its radio emission. Stratified MHD disk winds could therefore serve as a unique means to understand and unify the diverse AGN outflows.

Stratified Magnetically Driven Accretion-disk Winds and Their Relations to Jets

NASA Astrophysics Data System (ADS)

Fukumura, Keigo; Tombesi, Francesco; Kazanas, Demosthenes; Shrader, Chris; Behar, Ehud; Contopoulos, Ioannis

2014-01-01

We explore the poloidal structure of two-dimensional magnetohydrodynamic (MHD) winds in relation to their potential association with the X-ray warm absorbers (WAs) and the highly ionized ultra-fast outflows (UFOs) in active galactic nuclei (AGNs), in a single unifying approach. We present the density n(r, θ), ionization parameter ξ(r, θ), and velocity structure v(r, θ) of such ionized winds for typical values of their fluid-to-magnetic flux ratio, F, and specific angular momentum, H, for which wind solutions become super-Alfvénic. We explore the geometrical shape of winds for different values of these parameters and delineate the values that produce the widest and narrowest opening angles of these winds, quantities necessary in the determination of the statistics of AGN obscuration. We find that winds with smaller H show a poloidal geometry of narrower opening angles with their Alfvén surface at lower inclination angles and therefore they produce the highest line of sight (LoS) velocities for observers at higher latitudes with the respect to the disk plane. We further note a physical and spatial correlation between the X-ray WAs and UFOs that form along the same LoS to the observer but at different radii, r, and distinct values of n, ξ, and v consistent with the latest spectroscopic data of radio-quiet Seyfert galaxies. We also show that, at least in the case of 3C 111, the winds' pressure is sufficient to contain the relativistic plasma responsible for its radio emission. Stratified MHD disk winds could therefore serve as a unique means to understand and unify the diverse AGN outflows.

Self-injection of electrons in a laser-wakefield accelerator by using longitudinal density ripple

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dahiya, Deepak; Sharma, A. K.; Sajal, Vivek

By introducing a longitudinal density ripple (periodic modulation in background plasma density), we demonstrate self-injection of electrons in a laser-wakefield accelerator. The wakefield driven plasma wave, in presence of density ripple excites two side band waves of same frequency but different wave numbers. One of these side bands, having smaller phase velocity compared to wakefield driven plasma wave, preaccelerates the background plasma electrons. Significant number of these preaccelerated electrons get trapped in the laser-wakefield and further accelerated to higher energies.

Sun-stirred Kraken Mare: Circulation in Titan's seas induced by solar heating and methane precipitation

NASA Astrophysics Data System (ADS)

Tokano, Tetsuya; Lorenz, Ralph D.

2016-05-01

Density-driven circulation in Titan's seas forced by solar heating and methane evaporation/precipitation is simulated by an ocean circulation model. If the sea is transparent to sunlight, solar heating can induce anti-clockwise gyres near the sea surface and clockwise gyres near the sea bottom. The gyres are in geostrophic balance between the radially symmetric pressure gradient force and Coriolis force. If instead the sea is turbid and most sunlight is absorbed near the sea surface, the sea gets stratified in warm seasons and the circulation remains weak. Precipitation causes compositional stratification of the sea to an extent that the sea surface temperature can be lower than the sea interior temperature without causing a convective overturning. Non-uniform precipitation can also generate a latitudinal gradient in the methane mole fraction and density, which drives a meridional overturning with equatorward currents near the sea surface and poleward currents near the sea bottom. However, gyres are more ubiquitous than meridional overturning.

Visualization and simulation of density driven convection in porous media using magnetic resonance imaging.

PubMed

Montague, James A; Pinder, George F; Gonyea, Jay V; Hipko, Scott; Watts, Richard

2018-05-01

Magnetic resonance imaging is used to observe solute transport in a 40cm long, 26cm diameter sand column that contained a central core of low permeability silica surrounded by higher permeability well-sorted sand. Low concentrations (2.9g/L) of Magnevist, a gadolinium based contrast agent, produce density driven convection within the column when it starts in an unstable state. The unstable state, for this experiment, exists when higher density contrast agent is present above the lower density water. We implement a numerical model in OpenFOAM to reproduce the observed fluid flow and transport from a density difference of 0.3%. The experimental results demonstrate the usefulness of magnetic resonance imaging in observing three-dimensional gravity-driven convective-dispersive transport behaviors in medium scale experiments. Copyright © 2017 Elsevier B.V. All rights reserved.

Sensitivity of Rogue Waves Predictions to the Oceanic Stratification

NASA Astrophysics Data System (ADS)

Guo, Qiuchen; Alam, Mohammad-Reza

2014-11-01

Oceanic rogue waves are short-lived very large amplitude waves (a giant crest typically followed or preceded by a deep trough) that appear and disappear suddenly in the ocean causing damages to ships and offshore structures. Assuming that the state of the ocean at the present time is perfectly known, then the upcoming rogue waves can be predicted via numerically solving the equations that govern the evolution of the waves. The state of the art radar technology can now provide accurate wave height measurement over large spatial domains and when combined with advanced wave-field reconstruction techniques together render deterministic details of the current state of the ocean (i.e. surface elevation and velocity field) at any given moment of the time with a very high accuracy. The ocean water density is, however, stratified (mainly due to the salinity and temperature differences). This density stratification, with today's technology, is very difficult to be measured accurately. As a result in most predictive schemes these density variations are neglected. While the overall effect of the stratification on the average state of the ocean may not be significant, here we show that these density variations can strongly affect the prediction of oceanic rogue waves. Specifically, we consider a broadband oceanic spectrum in a two-layer density stratified fluid, and study via extensive statistical analysis the effects of strength of the stratification (difference between densities) and the depth of the thermocline on the prediction of upcoming rogue waves.

Erosion and deposition by supercritical density flows during channel avulsion and backfilling: Field examples from coarse-grained deepwater channel-levée complexes (Sandino Forearc Basin, southern Central America)

NASA Astrophysics Data System (ADS)

Lang, Jörg; Brandes, Christian; Winsemann, Jutta

2017-03-01

Erosion and deposition by supercritical density flows can strongly impact the facies distribution and architecture of submarine fans. Field examples from coarse-grained channel-levée complexes from the Sandino Forearc Basin (southern Central America) show that cyclic-step and antidune deposits represent common sedimentary facies of these depositional systems and relate to the different stages of avulsion, bypass, levée construction and channel backfilling. During channel avulsion, large-scale scour-fill complexes (18 to 29 m deep, 18 to 25 m wide, 60 to > 120 m long) were incised by supercritical density flows. The multi-storey infill of the large-scale scour-fill complexes comprises amalgamated massive, normally coarse-tail graded or widely spaced subhorizontally stratified conglomerates and pebbly sandstones, interpreted as deposits of the hydraulic-jump zone of cyclic steps. The large-scale scour-fill complexes can be distinguished from small-scale channel fills based on the preservation of a steep upper margin and a coarse-grained infill comprising mainly amalgamated hydraulic-jump zone deposits. Channel fills include repeated successions deposited by cyclic steps with superimposed antidunes. The deposits of the hydraulic-jump zone of cyclic steps comprise regularly spaced scours (0.2 to 2.6 m deep, 0.8 to 23 m long) infilled by intraclast-rich conglomerates or pebbly sandstones, displaying normal coarse-tail grading or backsets. These deposits are laterally and vertically associated with subhorizontally stratified, low-angle cross-stratified or sinusoidally stratified sandstones and pebbly sandstones, which were deposited by antidunes on the stoss side of the cyclic steps during flow re-acceleration. The field examples indicate that so-called spaced stratified deposits may commonly represent antidune deposits with varying stratification styles controlled by the aggradation rate, grain-size distribution and amalgamation. The deposits of small-scale cyclic steps with superimposed antidunes form fining-upwards successions with decreasing antidune wavelengths, indicating waning flows. Such cyclic step-antidune successions form the characteristic basal infill of mid-fan channels, and are inferred to be related to successive supercritical high-density turbidity flows triggered by retrogressive slope failures.

Turbidity current with a roof: Direct numerical simulation of self-stratified turbulent channel flow driven by suspended sediment

NASA Astrophysics Data System (ADS)

Cantero, Mariano I.; Balachandar, S.; Cantelli, Alessandro; Pirmez, Carlos; Parker, Gary

2009-03-01

In this work we present direct numerical simulations (DNS) of sediment-laden channel flows. In contrast to previous studies, where the flow has been driven by a constant, uniform pressure gradient, our flows are driven by the excess density imposed by suspended sediment. This configuration provides a simplified model of a turbidity current and is thus called the turbidity current with a roof configuration. Our calculations elucidate with DNS for the first time several fascinating features of sediment-laden flows, which may be summarized as follows. First, the presence of sediment breaks the symmetry of the flow because of a tendency to self-stratify. More specifically, this self-stratification is manifested in terms of a Reynolds-averaged suspended sediment concentration that declines in the upward normal direction and a Reynolds-averaged velocity profile with a maximum that is below the channel centerline. Second, this self-stratification damps the turbulence, particularly near the bottom wall. Two regimes are observed, one in which the flow remains turbulent but the level of turbulence is reduced and another in which the flow relaminarizes in a region near the bottom wall, i.e., bed. Third, the analysis allows the determination of a criterion for the break between these two regimes in terms of an appropriately defined dimensionless settling velocity. The results provide guidance for the improvement of Reynolds-averaged closures for turbulent flow in regard to stratification effects. Although the analysis reported here is not performed at the scale of large oceanic turbidity currents, which have sufficiently large Reynolds numbers to be inaccessible via DNS at this time, the implication of flow relaminarization is of considerable importance. Even a swift oceanic turbidity current which at some point crosses the threshold into the regime of relaminarization may lose the capacity to reentrain sediment that settles on the bed and thus may quickly die as it loses its driving force.

The Earth's mantle in a microwave oven: thermal convection driven by a heterogeneous distribution of heat sources

NASA Astrophysics Data System (ADS)

Fourel, Loïc; Limare, Angela; Jaupart, Claude; Surducan, Emanoil; Farnetani, Cinzia G.; Kaminski, Edouard C.; Neamtu, Camelia; Surducan, Vasile

2017-08-01

Convective motions in silicate planets are largely driven by internal heat sources and secular cooling. The exact amount and distribution of heat sources in the Earth are poorly constrained and the latter is likely to change with time due to mixing and to the deformation of boundaries that separate different reservoirs. To improve our understanding of planetary-scale convection in these conditions, we have designed a new laboratory setup allowing a large range of heat source distributions. We illustrate the potential of our new technique with a study of an initially stratified fluid involving two layers with different physical properties and internal heat production rates. A modified microwave oven is used to generate a uniform radiation propagating through the fluids. Experimental fluids are solutions of hydroxyethyl cellulose and salt in water, such that salt increases both the density and the volumetric heating rate. We determine temperature and composition fields in 3D with non-invasive techniques. Two fluorescent dyes are used to determine temperature. A Nd:YAG planar laser beam excites fluorescence, and an optical system, involving a beam splitter and a set of colour filters, captures the fluorescence intensity distribution on two separate spectral bands. The ratio between the two intensities provides an instantaneous determination of temperature with an uncertainty of 5% (typically 1K). We quantify mixing processes by precisely tracking the interfaces separating the two fluids. These novel techniques allow new insights on the generation, morphology and evolution of large-scale heterogeneities in the Earth's lower mantle.

Soil-borne pathogens restrict the recruitment of a subtropical tree: a distance-dependent effect.

PubMed

Xu, Meng; Wang, Yongfan; Liu, Yu; Zhang, Zhiming; Yu, Shixiao

2015-03-01

The Janzen-Connell hypothesis suggests that density- and/or distance-dependent juvenile mortality driven by host-specific natural enemies can explain high species diversity in tropical forests. However, such density and distance effects may not occur simultaneously and may not be driven by the same mechanism. Also, reports of attempts to identify and quantify the differences between these processes in tropical forests are scarce. In a primary subtropical forest in China, we (1) experimentally examined the relative influence of the distance to parent trees vs. conspecific seedling density on mortality patterns in Engelhardia fenzelii, (2) tested the role of soil-borne pathogens in driving density- or distance-dependent processes that cause seedling mortality, and (3) inspected the susceptibilities of different tree species to soil biota of E. fenzelii and the effects of soil biota from different tree species on E. fenzelii. The results from these field experiments showed that distance- rather than density-dependent processes driven by soil pathogens strongly affect the seedling survival of this species in its first year. We also observed increased survival of a fungicide treatment for E. fenzelii seedlings in the parent soil but not for the seedlings of the other three species in the E. fenzelii parent soil, or for E. fenzelii seedlings in the parent soil of three other species. This study illustrates how the distance-dependent pattern of seedling recruitment for this species is driven by soil pathogens, a mechanism that likely restricts the dominance of this abundant species.

New Numerical Approaches for Modeling Thermochemical Convection in a Compositionally Stratified Fluid

NASA Astrophysics Data System (ADS)

Puckett, E. G.; Turcotte, D. L.; He, Y.; Lokavarapu, H. V.; Robey, J.; Kellogg, L. H.

2017-12-01

Geochemical observations of mantle-derived rocks favor a nearly homogeneous upper mantle, the source of mid-ocean ridge basalts (MORB), and heterogeneous lower mantle regions.Plumes that generate ocean island basalts are thought to sample the lower mantle regions and exhibit more heterogeneity than MORB.These regions have been associated with lower mantle structures known as large low shear velocity provinces below Africa and the South Pacific.The isolation of these regions is attributed to compositional differences and density stratification that, consequently, have been the subject of computational and laboratory modeling designed to determine the parameter regime in which layering is stable and understanding how layering evolves.Mathematical models of persistent compositional interfaces in the Earth's mantle may be inherently unstable, at least in some regions of the parameter space relevant to the mantle.Computing approximations to solutions of such problems presents severe challenges, even to state-of-the-art numerical methods.Some numerical algorithms for modeling the interface between distinct compositions smear the interface at the boundary between compositions, such as methods that add numerical diffusion or `artificial viscosity' in order to stabilize the algorithm. We present two new algorithms for maintaining high-resolution and sharp computational boundaries in computations of these types of problems: a discontinuous Galerkin method with a bound preserving limiter and a Volume-of-Fluid interface tracking algorithm.We compare these new methods with two approaches widely used for modeling the advection of two distinct thermally driven compositional fields in mantle convection computations: a high-order accurate finite element advection algorithm with entropy viscosity and a particle method.We compare the performance of these four algorithms on three problems, including computing an approximation to the solution of an initially compositionally stratified fluid at Ra = 105 with buoyancy numbers {B} that vary from no stratification at B = 0 to stratified flow at large B.

Activity-induced instability of phonons in 1D microfluidic crystals.

PubMed

Tsang, Alan Cheng Hou; Shelley, Michael J; Kanso, Eva

2018-02-14

One-dimensional crystals of passively-driven particles in microfluidic channels exhibit collective vibrational modes reminiscent of acoustic 'phonons'. These phonons are induced by the long-range hydrodynamic interactions among the particles and are neutrally stable at the linear level. Here, we analyze the effect of particle activity - self-propulsion - on the emergence and stability of these phonons. We show that the direction of wave propagation in active crystals is sensitive to the intensity of the background flow. We also show that activity couples, at the linear level, transverse waves to the particles' rotational motion, inducing a new mode of instability that persists in the limit of large background flow, or, equivalently, vanishingly small activity. We then report a new phenomenon of phonons switching back and forth between two adjacent crystals in both passively-driven and active systems, similar in nature to the wave switching observed in quantum mechanics, optical communication, and density stratified fluids. These findings could have implications for the design of commercial microfluidic systems and the self-assembly of passive and active micro-particles into one-dimensional structures.

Building a Better Model: A Comprehensive Breast Cancer Risk Model Incorporating Breast Density to Stratify Risk and Apply Resources

DTIC Science & Technology

2014-10-01

variability with well trained readers. Figure 7: comparison between the PD (percent density using Cumulus area) and the automatic PD. The...evaluation of outlier correction, comparison of several different software methods, precision measurement, and evaluation of variation by mammography...chart review for selected cases (month 4-6). Comparison of information from the Breast Cancer Database and medical records showed good consistency

Effects of density stratification on the frequencies of the inertial-gravity modes of the Earth's fluid core

NASA Astrophysics Data System (ADS)

Seyed-Mahmoud, B.; Moradi, A.; Kamruzzaman, M.; Naseri, H.

2015-08-01

The Earth's outer core is a rotating ellipsoidal shell of compressible, stratified and self-gravitating fluid. As such, in the treatment of geophysical problems a realistic model of this body needs to be considered. In this work, we consider compressible and stratified fluid core models with different stratification parameters, related to the local Brunt-Väisälä frequency, in order to study the effects of the core's density stratification on the frequencies of some of the inertial-gravity modes of this body. The inertial-gravity modes of the core are free oscillations with periods longer than 12 hr. Historically, an incompressible and homogeneous fluid is considered to study these modes and analytical solutions are known for the frequencies and the displacement eigenfunctions of a spherical model. We show that for a compressible and stratified spherical core model the effects of non-neutral density stratification may be significant, and the frequencies of these modes may change from model to model. For example, for a spherical core model the frequency of the spin-over mode, the (2, 1, 1) mode, is unaffected while that of the (4, 1, 1) mode is changed from -0.410 for the Poincaré core model to -0.434, -0.447 and -0.483 for core models with the stability parameter β = -0.001, -0.002 and -0.005, respectively, a maximum change of about 18 per cent when β = -0.005. Our results also show that for small stratification parameter, |β| ≤ 0.005, the frequency of an inertial-gravity mode is a nearly linear function of β but the slope of the line is different for different modes, and that the effects of density stratification on the frequency of a mode is likely related to its spatial structure, which remains the same in different Earth models. We also compute the frequencies of some of the modes of the `PREM' (spherical shell) core model and show that the frequencies of these modes may also be significantly affected by non-zero β.

Metal enrichment of the intracluster medium: SN-driven galactic winds

NASA Astrophysics Data System (ADS)

Baumgartner, V.; Breitschwerdt, D.

2009-12-01

% We investigate the role of supernova (SN)-driven galactic winds in the chemical enrichment of the intracluster medium (ICM). Such outflows on galactic scales have their origin in huge star forming regions and expel metal enriched material out of the galaxies into their surroundings as observed, for example, in the nearby starburst galaxy NGC 253. As massive stars in OB-associations explode sequentially, shock waves are driven into the interstellar medium (ISM) of a galaxy and merge, forming a superbubble (SB). These SBs expand in a direction perpendicular to the disk plane following the density gradient of the ISM. We use the 2D analytical approximation by Kompaneets (1960) to model the expansion of SBs in an exponentially stratified ISM. This is modified in order to describe the sequence of SN-explosions as a time-dependent process taking into account the main-sequence life-time of the SN-progenitors and using an initial mass function to get the number of massive stars per mass interval. The evolution of the bubble in space and time is calculated analytically, from which the onset of Rayleigh-Taylor instabilities in the shell can be determined. In its further evolution, the shell will break up and high-metallicity gas will be ejected into the halo of the galaxy and even into the ICM. We derive the number of stars needed for blow-out depending on the scale height and density of the ambient medium, as well as the fraction of alpha- and iron peak elements contained in the hot gas. Finally, the amount of metals injected by Milky Way-type galaxies to the ICM is calculated confirming the importance of this enrichment process.

Remote Sensing, Sampling and Simulation Applications in Analyses of Insect Dispersion and Abundance in Cotton

Treesearch

J. L. Willers; J. M. McKinion; J. N. Jenkins

2006-01-01

Simulation was employed to create stratified simple random samples of different sample unit sizes to represent tarnished plant bug abundance at different densities within various habitats of simulated cotton fields. These samples were used to investigate dispersion patterns of this cotton insect. It was found that the assessment of spatial pattern varied as a function...

Empirical relationships among resilience indicators on Micronesian reefs

USGS Publications Warehouse

Mumby, P.J.; Bejarano, S.; Golbuu, Y.; Steneck, R.S.; Arnold, S.N.; van Woesik, R.; Friedlander, A.M.

2013-01-01

A process-orientated understanding of ecosystems usually starts with an exploratory analysis of empirical relationships among potential drivers and state variables. While relationships among herbivory, algal cover, and coral recruitment, have been explored in the Caribbean, the nature of such relationships in the Pacific appears to be variable or unclear. Here, we examine potential drivers structuring the benthos and herbivorous fish assemblages of outer-shelf reefs in Micronesia (Palau, Guam and Pohnpei). Surveys were stratified by wave exposure and protection from fishing. High biomass of most herbivores was favoured by high wave exposure. High abundance of large-bodied scarids was associated with low turf abundance, high coral cover, and marine reserves. The remaining herbivores were more abundant in reefs with low coral cover, possibly because space and hence food limitation occur in high-coral-cover reefs. Rugosity had no detectable effect on herbivorous fish abundance once differences in exposure and coral cover were accounted for. At identical depths, high wave exposure was associated with greater volumes (cover × canopy height) of macroalgae and algal turfs, which most likely resulted from high primary productivity driven by flow. In exposed areas, macroalgal cover declined as the acanthurid biomass increased. The volume of algal turfs was negatively associated with coral cover and herbivore biomass. In turn, high coral cover and herbivore biomass are likely to intensify grazing. The density of juvenile corals was variable where macroalgal cover was low but was confined to lower densities where macroalgal cover was high. High coral cover and density of juvenile corals were favoured in sheltered habitats. While a weak positive relationship was found between scarid biomass and juvenile coral density, we hypothesise that high scarid densities may hinder juvenile density through increased corallivory. New hypotheses emerged that will help clarify the role of acanthurids, wave exposure, and corallivory in driving the recovery of Pacific coral communities.

Experimental study of stratified jet by simultaneous measurements of velocity and density fields

NASA Astrophysics Data System (ADS)

Xu, Duo; Chen, Jun

2012-07-01

Stratified flows with small density difference commonly exist in geophysical and engineering applications, which often involve interaction of turbulence and buoyancy effect. A combined particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) system is developed to measure the velocity and density fields in a dense jet discharged horizontally into a tank filled with light fluid. The illumination of PIV particles and excitation of PLIF dye are achieved by a dual-head pulsed Nd:YAG laser and two CCD cameras with a set of optical filters. The procedure for matching refractive indexes of two fluids and calibration of the combined system are presented, as well as a quantitative analysis of the measurement uncertainties. The flow structures and mixing dynamics within the central vertical plane are studied by examining the averaged parameters, turbulent kinetic energy budget, and modeling of momentum flux and buoyancy flux. At downstream, profiles of velocity and density display strong asymmetry with respect to its center. This is attributed to the fact that stable stratification reduces mixing and unstable stratification enhances mixing. In stable stratification region, most of turbulence production is consumed by mean-flow convection, whereas in unstable stratification region, turbulence production is nearly balanced by viscous dissipation. Experimental data also indicate that at downstream locations, mixing length model performs better in mixing zone of stable stratification regions, whereas in other regions, eddy viscosity/diffusivity models with static model coefficients represent effectively momentum and buoyancy flux terms. The measured turbulent Prandtl number displays strong spatial variation in the stratified jet.

Influences of source condition and dissolution on bubble plume in a stratified environment

NASA Astrophysics Data System (ADS)

Chu, Shigan; Prosperetti, Andrea

2017-11-01

A cross-sectionally averaged model is used to study a bubble plume rising in a stratified quiescent liquid. Scaling analyses for the peel height, at which the plume momentum vanishes, and the neutral height, at which its average density equals the ambient density, are presented. Contrary to a widespread practice in the literature, it is argued that the neutral height cannot be identified with the experimentally reported intrusion height. Recognizing this difference provides an explanation of the reason why the intrusion height is found so frequently to lie so much above predictions, and brings the theoretical results in line with observations. The mathematical model depends on three dimensionless parameters, some of which are related to the inlet conditions at the plume source. Their influence on the peel and neutral heights is illustrated by means of numerical results. Aside from the source parameters, we incorporate dissolution of bubbles and the corresponding density change of plume into the model. Contrary to what's documented in literature, density change of plume due to dissolution plays an important role in keeping the total buoyancy of plume, thus alleviating the rapid decrease of peel height because of dissolution.

Direct numerical simulation of incompressible acceleration-driven variable-density turbulence

NASA Astrophysics Data System (ADS)

Gat, Ilana; Matheou, Georgios; Chung, Daniel; Dimotakis, Paul

2015-11-01

Fully developed turbulence in variable-density flow driven by an externally imposed acceleration field, e.g., gravity, is fundamental in many applications, such as inertial confinement fusion, geophysics, and astrophysics. Aspects of this turbulence regime are poorly understood and are of interest to fluid modeling. We investigate incompressible acceleration-driven variable-density turbulence by a series of direct numerical simulations of high-density fluid in-between slabs of low-density fluid, in a triply-periodic domain. A pseudo-spectral numerical method with a Helmholtz-Hodge decomposition of the pressure field, which ensures mass conservation, is employed, as documented in Chung & Pullin (2010). A uniform dynamic viscosity and local Schmidt number of unity are assumed. This configuration encapsulates a combination of flow phenomena in a temporally evolving variable-density shear flow. Density ratios up to 10 and Reynolds numbers in the fully developed turbulent regime are investigated. The temporal evolution of the vertical velocity difference across the shear layer, shear-layer growth, mean density, and Reynolds number are discussed. Statistics of Lagrangian accelerations of fluid elements and of vorticity as a function of the density ratio are also presented. This material is based upon work supported by the AFOSR, the DOE, the NSF GRFP, and Caltech.

Different phases of a system of hard rods on three dimensional cubic lattice

NASA Astrophysics Data System (ADS)

Vigneshwar, N.; Dhar, Deepak; Rajesh, R.

2017-11-01

We study the different phases of a system of monodispersed hard rods of length k on a cubic lattice, using an efficient cluster algorithm able to simulate densities close to the fully-packed limit. For k≤slant 4 , the system is disordered at all densities. For k=5, 6 , we find a single density-driven transition, from a disordered phase to high density layered-disordered phase, in which the density of rods of one orientation is strongly suppressed, breaking the system into weakly coupled layers. Within a layer, the system is disordered. For k ≥slant 7 , three density-driven transitions are observed numerically: isotropic to nematic to layered-nematic to layered-disordered. In the layered-nematic phase, the system breaks up into layers, with nematic order in each layer, but very weak correlation between the ordering directions of different layers. We argue that the layered-nematic phase is a finite-size effect, and in the thermodynamic limit, the nematic phase will have higher entropy per site. We expect the systems of rods in four and higher dimensions will have a qualitatively similar phase diagram.

Stationary States of Boundary Driven Exclusion Processes with Nonreversible Boundary Dynamics

NASA Astrophysics Data System (ADS)

Erignoux, C.; Landim, C.; Xu, T.

2018-05-01

We prove a law of large numbers for the empirical density of one-dimensional, boundary driven, symmetric exclusion processes with different types of non-reversible dynamics at the boundary. The proofs rely on duality techniques.

The effect of different methods to compute N on estimates of mixing in stratified flows

NASA Astrophysics Data System (ADS)

Fringer, Oliver; Arthur, Robert; Venayagamoorthy, Subhas; Koseff, Jeffrey

2017-11-01

The background stratification is typically well defined in idealized numerical models of stratified flows, although it is more difficult to define in observations. This may have important ramifications for estimates of mixing which rely on knowledge of the background stratification against which turbulence must work to mix the density field. Using direct numerical simulation data of breaking internal waves on slopes, we demonstrate a discrepancy in ocean mixing estimates depending on the method in which the background stratification is computed. Two common methods are employed to calculate the buoyancy frequency N, namely a three-dimensionally resorted density field (often used in numerical models) and a locally-resorted vertical density profile (often used in the field). We show that how N is calculated has a significant effect on the flux Richardson number Rf, which is often used to parameterize turbulent mixing, and the turbulence activity number Gi, which leads to errors when estimating the mixing efficiency using Gi-based parameterizations. Supported by ONR Grant N00014-08-1-0904 and LLNL Contract DE-AC52-07NA27344.

Transition from Selective Withdrawal to Light Layer Entrainment in an Oil-Water System

NASA Astrophysics Data System (ADS)

Hartenberger, Joel; O'Hern, Timothy; Webb, Stephen; James, Darryl

2010-11-01

Selective withdrawal refers to the selective removal of fluid of one density without entraining an adjacent fluid layer of a different density. Most prior literature has examined removal of the lower density fluid and the transition to entraining the higher density fluid. In the present experiments, a higher density liquid is removed through a tube that extends just below its interface with a lower density fluid. The critical depth for a given flow rate at which the liquid-liquid interface transitions to entrain the lighter fluid was measured. Experiments were performed for a range of different light layer silicone oils and heavy layer water or brine, covering a range of density and viscosity ratios. Applications include density-stratified reservoirs and brine removal from oil storage caverns. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Density-Difference-Driven Optimized Embedding Potential Method To Study the Spectroscopy of Br₂ in Water Clusters.

PubMed

Roncero, Octavio; Aguado, Alfredo; Batista-Romero, Fidel A; Bernal-Uruchurtu, Margarita I; Hernández-Lamoneda, Ramón

2015-03-10

A variant of the density difference driven optimized embedding potential (DDD-OEP) method, proposed by Roncero et al. (J. Chem. Phys. 2009, 131, 234110), has been applied to the calculation of excited states of Br2 within small water clusters. It is found that the strong interaction of Br2 with the lone electronic pair of the water molecules makes necessary to optimize specific embedding potentials for ground and excited electronic states, separately and using the corresponding densities. Diagnosis and convergence studies are presented with the aim of providing methods to be applied for the study of chromophores in solution. Also, some preliminary results obtained for the study of electronic states of Br2 in clathrate cages are presented.

Experimental observation of the stratified electrothermal instability on aluminum with thickness greater than a skin depth

NASA Astrophysics Data System (ADS)

Hutchinson, T. M.; Awe, T. J.; Bauer, B. S.; Yates, K. C.; Yu, E. P.; Yelton, W. G.; Fuelling, S.

2018-05-01

A direct observation of the stratified electrothermal instability on the surface of thick metal is reported. Aluminum rods coated with 70 μ m Parylene-N were driven to 1 MA in 100 ns , with the metal thicker than the skin depth. The dielectric coating suppressed plasma formation, enabling persistent observation of discrete azimuthally correlated stratified thermal perturbations perpendicular to the current whose wave numbers, k , grew exponentially with rate γ (k ) =0.06 n s-1-(0.4 n s-1μ m2ra d-2 ) k2 in ˜1 g /c m3 , ˜7000 K aluminum.

Experimental Observation of the Stratified Electrothermal Instability on Aluminum with Thickness Greater than a Skin Depth

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hutchinson, Trevor M.; Hutchinson, Trevor M.; Awe, Thomas James

The first direct observation of the stratified electrothermal instability on the surface of thick metal is reported. Aluminum rods coated with 70 μm Parylene-N were driven to 1 MA in approximately 100 ns, with the metal thicker than the skin depth. The dielectric coating suppressed plasma formation, enabling persistent observation of discrete azimuthally-correlated stratified structures perpendicular to the current. Strata amplitudes grow rapidly, while their Fourier spectrum shifts toward longer wavelength. Assuming blackbody emission, radiometric calculations indicate strata are temperature perturbations that grow exponentially with rate γ = 0.04 ns -1 in 3000- 10,000 K aluminum.

Unsteady Shear Disturbances Within a Two Dimensional Stratified Flow

NASA Technical Reports Server (NTRS)

Yokota, Jeffrey W.

1992-01-01

The origin and evolution of shear disturbances within a stratified, inviscid, incompressible flow are investigated numerically by a Clebsch/Weber decomposition based scheme. In contrast to homogeneous flows, within which vorticity can be redistributed but not generated, the presence of a density stratification can render an otherwise irrotational flow vortical. In this work, a kinematic decomposition of the unsteady Euler equations separates the unsteady velocity field into rotational and irrotational components. The subsequent evolution of these components is used to study the influence various velocity disturbances have on both stratified and homogeneous flows. In particular, the flow within a two-dimensional channel is used to investigate the evolution of rotational disturbances, generated or convected, downstream from an unsteady inflow condition. Contrasting simulations of both stratified and homogeneous flows are used to distinguish between redistributed inflow vorticity and that which is generated by a density stratification.

Sedimentology and geomorphology of the deposits from the August 2006 pyroclastic density currents at Tungurahua volcano, Ecuador.

PubMed

Douillet, Guilhem Amin; Tsang-Hin-Sun, Ève; Kueppers, Ulrich; Letort, Jean; Pacheco, Daniel Alejandro; Goldstein, Fabian; Von Aulock, Felix; Lavallée, Yan; Hanson, Jonathan Bruce; Bustillos, Jorge; Robin, Claude; Ramón, Patricio; Hall, Minard; Dingwell, Donald B

The deposits of the pyroclastic density currents from the August 2006 eruption of Tungurahua show three facies associations depending on the topographic setting: the massive, proximal cross-stratified, and distal cross-stratified facies. (1) The massive facies is confined to valleys on the slopes of the volcano. It contains clasts of >1 m diameter to fine ash material, is massive, and interpreted as deposited from dense pyroclastic flows. Its surface can exhibit lobes and levees covered with disk-shaped and vesicular large clasts. These fragile large clasts must have rafted at the surface of the flows all along the path in order to be preserved, and thus imply a sharp density boundary near the surface of these flows. (2) The proximal cross-stratified facies is exposed on valley overbanks on the upper part of the volcano and contains both massive coarse-grained layers and cross-stratified ash and lapilli bedsets. It is interpreted as deposited from (a) dense pyroclastic flows that overflowed the gentle ridges of valleys of the upper part of the volcano and (b) dilute pyroclastic density currents created from the dense flows by the entrainment of air on the steep upper flanks. (3) The distal cross-stratified facies outcrops as spatially limited, isolated, and wedge-shaped bodies of cross-stratified ash deposits located downstream of cliffs on valleys overbanks. It contains numerous aggrading dune bedforms, whose crest orientations reveal parental flow directions. A downstream decrease in the size of the dune bedforms, together with a downstream fining trend in the grain size distribution are observed on a 100-m scale. This facies is interpreted to have been deposited from dilute pyroclastic density currents with basal tractional boundary layers. We suggest that the parental flows were produced from the dense flows by entrainment of air at cliffs, and that these diluted currents might rapidly deposit through "pneumatic jumps". Three modes are present in the grain size distribution of all samples independently of the facies, which further supports the interpretation that all three facies derive from the same initial flows. This study emphasizes the influence of topography on small volume pyroclastic density currents, and the importance of flow transformation and flow-stripping processes.

Cyclic steps and superimposed antidune deposits: important elements of coarse-grained deepwater channel-levée complexes

NASA Astrophysics Data System (ADS)

Lang, Joerg; Brandes, Christian; Winsemann, Jutta

2017-04-01

The facies distribution and architecture of submarine fans can be strongly impacted by erosion and deposition by supercritical density flows. We present field examples from the Sandino Forearc Basin (southern Central America), where cyclic-step and antidune deposits represent important sedimentary facies of coarse-grained channel-levée complexes. These bedforms occur in all sub-environments of the depositional systems and relate to the different stages of avulsion, bypass, levée construction and channel backfilling. Large-scale scours (18 to 29 m deep, 18 to 25 m wide, 60 to >120 m long) with an amalgamated infill, comprising massive, normally coarse-tail graded or spaced subhorizontally stratified conglomerates and pebbly sandstones, are interpreted as deposits of the hydraulic-jump zone of cyclic steps. These cyclic steps probably formed during avulsion, when high-density flows were routed into the evolving channel. The large-scale scour fills can be distinguished from small-scale channel fills based on the preservation of a steep upper margin and a coarse-grained infill comprising mainly amalgamated hydraulic-jump deposits. Channel fills include repetitive successions deposited by cyclic steps with superimposed antidunes. The hydraulic-jump zone of cyclic-step deposits comprises regularly spaced scours (0.2 to 2.6 m deep, 0.8 to 23 m wide), which are infilled by intraclast-rich conglomerates or pebbly sandstones and display normal coarse-tail grading or backsets. Laterally and vertically these deposits are associated with subhorizontally stratified, low-angle cross-stratified or sinusoidal stratified pebbly sandstones and sandstones (wavelength 0.5 to 18 m), interpreted as representing antidune deposits formed on the stoss-side of the cyclic steps during flow re-acceleration. The field examples indicate that so-called crudely or spaced stratified deposits may commonly represent antidune deposits with varying stratification styles controlled by the aggradation rate, grain-size distribution and amalgamation. The deposits of small-scale cyclic steps with superimposed antidunes form fining upwards successions with decreasing antidune wavelengths. Such cyclic step-antidune successions are the characteristic basal infill of channels, probably related to supercritical high-density turbidity flows triggered by retrogressive slope failures.

An affordable and accurate conductivity probe for density measurements in stratified flows

NASA Astrophysics Data System (ADS)

Carminati, Marco; Luzzatto-Fegiz, Paolo

2015-11-01

In stratified flow experiments, conductivity (combined with temperature) is often used to measure density. The probes typically used can provide very fine spatial scales, but can be fragile, expensive to replace, and sensitive to environmental noise. A complementary instrument, comprising a low-cost conductivity probe, would prove valuable in a wide range of applications where resolving extremely small spatial scales is not needed. We propose using micro-USB cables as the actual conductivity sensors. By removing the metallic shield from a micro-B connector, 5 gold-plated microelectrodes are exposed and available for 4-wire measurements. These have a cell constant ~550m-1, an intrinsic thermal noise of at most 30pA/Hz1/2, as well as sub-millisecond time response, making them highly suitable for many stratified flow measurements. In addition, we present the design of a custom electronic board (Arduino-based and Matlab-controlled) for simultaneous acquisition from 4 sensors, with resolution (in conductivity, and resulting density) exceeding the performance of typical existing probes. We illustrate the use of our conductivity-measuring system through stratified flow experiments, and describe plans to release simple instructions to construct our complete system for around 200.

Periodicity of the density wake past a vortex ring in a stratified liquid

NASA Astrophysics Data System (ADS)

Prokhorov, V.

2009-04-01

Spatial coherent structure of the density wake past a vortex ring moving horizontally in viscid stratified liquid is experimentally revealed. It follows from analysis that repetition period of the structure is determined by rotation radial frequency (or mean vorticity) of the vortex core and toward speed of the vortex ring. The wake formation of the ring is considered in respect to vorticity shedding which produces velocity disturbances in ambient medium. In case of stratified liquid velocity fluctuations, in their turn, cause density field distortion. This process is superimposed by vortex core oscillations, and, in result, vorticity shedding will be not monotonous but modulated at some frequency. So, the density wake is periodically structured, and the spatial period is defined by intrinsic frequency of the core and forward speed of the ring. To support analysis, experiments were conducted in which vortex rings excited by spring-piston generator were observed with high-sensitive Schlieren instrument and computer-controlled camera. Experimental tank was filled with salt-stratified water of constant buoyancy period, vortex ring velocities range from 3 to 16 cm/s. Spatial period is derived from schlieren image using two independent methods, both 2D spectral analysis and geometry calculations of the vortex core. Spatial periods and vortex intrinsic frequencies calculated by both algorithms are in good agreement; they vary in power lows depending on vortex speed

DOE Office of Scientific and Technical Information (OSTI.GOV)

Borovsky, Joseph E; Cayton, Thomas E; Denton, Michael H

Electron flux measurements from 7 satellites in geosynchronous orbit from 1990-2007 are fit with relativistic bi-Maxwellians, yielding a number density n and temperature T description of the outer electron radiation belt. For 54.5 spacecraft years of measurements the median value ofn is 3.7x10-4 cm-3 and the median value ofT is 142 keY. General statistical properties of n, T, and the 1.1-1.5 MeV flux J are investigated, including local-time and solar-cycle dependencies. Using superposed-epoch analysis triggered on storm onset, the evolution of the outer electron radiation belt through high-speed-steam-driven storms is investigated. The number density decay during the calm before themore » storm is seen, relativistic-electron dropouts and recoveries from dropout are investigated, and the heating of the outer electron radiation belt during storms is examined. Using four different triggers (SSCs, southward-IMF CME sheaths, southward-IMF magnetic clouds, and minimum Dst), CME-driven storms are analyzed with superposed-epoch techniques. For CME-driven storms an absence of a density decay prior to storm onset is found, the compression of the outer electron radiation belt at time of SSC is analyzed, the number-density increase and temperature decrease during storm main phase is seen, and the increase in density and temperature during storm recovery phase is observed. Differences are found between the density-temperature and the flux descriptions, with more information for analysis being available in the density-temperature description.« less

Normal Mode Derived Models of the Physical Properties of Earth's Outer Core

NASA Astrophysics Data System (ADS)

Irving, J. C. E.; Cottaar, S.; Lekic, V.; Wu, W.

2017-12-01

Earth's outer core, the largest reservoir of metal in our planet, is comprised of an iron alloy of an uncertain composition. Its dynamical behaviour is responsible for the generation of Earth's magnetic field, with convection driven both by thermal and chemical buoyancy fluxes. Existing models of the seismic velocity and density of the outer core exhibit some variation, and there are only a small number of models which aim to represent the outer core's density.It is therefore important that we develop a better understanding of the physical properties of the outer core. Though most of the outer core is likely to be well mixed, it is possible that the uppermost outer core is stably stratified: it may be enriched in light elements released during the growth of the solid, iron enriched, inner core; by elements dissolved from the mantle into the outer core; or by exsolution of compounds previously dissolved in the liquid metal which will eventually be swept into the mantle. The stratified layer may host MAC or Rossby waves and it could impede communication between the chemically differentiated mantle and outer core, including screening out some of the geodynamo's signal. We use normal mode center frequencies to estimate the physical properties of the outer core in a Bayesian framework. We estimate the mineral physical parameters needed to best produce velocity and density models of the outer core which are consistent with the normal mode observations. We require that our models satisfy realistic physical constraints. We create models of the outer core with and without a distinct uppermost layer and assess the importance of this region.Our normal mode-derived models are compared with observations of body waves which travel through the outer core. In particular, we consider SmKS waves which are especially sensitive to the uppermost outer core and are therefore an important way to understand the robustness of our models.

The propagation of the shock wave from a strong explosion in a plane-parallel stratified medium: the Kompaneets approximation

NASA Astrophysics Data System (ADS)

Olano, C. A.

2009-11-01

Context: Using certain simplifications, Kompaneets derived a partial differential equation that states the local geometrical and kinematical conditions that each surface element of a shock wave, created by a point blast in a stratified gaseous medium, must satisfy. Kompaneets could solve his equation analytically for the case of a wave propagating in an exponentially stratified medium, obtaining the form of the shock front at progressive evolutionary stages. Complete analytical solutions of the Kompaneets equation for shock wave motion in further plane-parallel stratified media were not found, except for radially stratified media. Aims: We aim to analytically solve the Kompaneets equation for the motion of a shock wave in different plane-parallel stratified media that can reflect a wide variety of astrophysical contexts. We were particularly interested in solving the Kompaneets equation for a strong explosion in the interstellar medium of the Galactic disk, in which, due to intense winds and explosions of stars, gigantic gaseous structures known as superbubbles and supershells are formed. Methods: Using the Kompaneets approximation, we derived a pair of equations that we call adapted Kompaneets equations, that govern the propagation of a shock wave in a stratified medium and that permit us to obtain solutions in parametric form. The solutions provided by the system of adapted Kompaneets equations are equivalent to those of the Kompaneets equation. We solved the adapted Kompaneets equations for shock wave propagation in a generic stratified medium by means of a power-series method. Results: Using the series solution for a shock wave in a generic medium, we obtained the series solutions for four specific media whose respective density distributions in the direction perpendicular to the stratification plane are of an exponential, power-law type (one with exponent k=-1 and the other with k =-2) and a quadratic hyperbolic-secant. From these series solutions, we deduced exact solutions for the four media in terms of elemental functions. The exact solution for shock wave propagation in a medium of quadratic hyperbolic-secant density distribution is very appropriate to describe the growth of superbubbles in the Galactic disk. Member of the Carrera del Investigador Científico del CONICET, Argentina.

Reproducibility of Respondent-Driven Sampling (RDS) in Repeat Surveys of Men Who have Sex with Men, Unguja, Zanzibar.

PubMed

Khatib, Ahmed; Haji, Shaaban; Khamis, Maryam; Said, Christen; Khalid, Farhat; Dahoma, Mohammed; Ali, Ameir; Othman, Asha; Welty, Susie; McFarland, Willi

2017-07-01

To assess the reproducibility of respondent-driven sampling (RDS) in obtaining comparable samples across two survey rounds, we conducted integrated bio-behavioral surveillance surveys (IBBSS) using RDS in 2007 and 2011 among men who have sex with men (MSM) on Unguja island in Zanzibar. Differences in the two rounds were assessed by comparing RDS-adjusted population estimates, stratified estimates, and bottleneck plots. Participants in the 2011 survey round were younger (31.4 vs. 9.9% under 19 years old, p < 0.001), more likely to have tested for HIV in the last year (53.7 vs. 10.6%, p < 0.001), and less likely to have injected drugs in the last 3 months (1.0 vs. 23.2%, p < 0.001) compared to participants in the 2007 round. HIV prevalence was 12.3% in 2007 compared to 2.6% in 2011 (p < 0.001). The difference in HIV prevalence persisted after stratifying and adjusting for known differences in the two surveys rounds. Bottleneck plots suggest that recruitment chains were "trapped" in the social networks of MSM who injected drugs to a greater extent in 2007 than in 2011. We conclude that the two rounds of RDS sampled different subsets of the MSM population on Unguja, particularly with respect to inclusion of MSM within the social networks of people who inject drugs. Findings underscore the need to evaluate the reproducibility of RDS in repeated rounds of IBBSS and to develop new sampling methods for key populations at high risk for HIV in order to track the epidemic, develop evidence-based prevention and care programs, and assess their impact.

RECONNECTION-DRIVEN DOUBLE LAYERS IN THE STRATIFIED PLASMA OF THE SOLAR TRANSITION REGION: SUPPLY OF HOT PLASMA INTO THE CORONA

DOE Office of Scientific and Technical Information (OSTI.GOV)

Singh, Nagendra

A novel mechanism for the supply of hot plasma into the corona from the chromosphere is suggested here; the mechanism involves collisionless magnetic reconnection (CMR) in the transition region (TR) followed by double layer (DL) formation in the enhanced expansion of the chromospheric cold plasma mixed with CMR-heated hot electrons. It is well known that (i) the CMR produces energetic electrons and (ii) DLs naturally form in expanding dense plasmas containing a minor population of hot electrons. We apply these plasma physics facts to the dynamics of stratified plasma in the TR. In the TR where densities fall below ∼10{supmore » 16} m{sup −3}, all collisional mean-free paths, electron–ion, ion–neutral, and electron–neutral, become long enough to render plasma collisionless at kinetic scale lengths, making CMR and DL formation possible. The DLs accelerate the chromospheric cold ions to energies comparable to the energy of the hot electrons. When the upflowing energized ions neutralized by the escaping hot electrons thermalize, the resulting hot tenuous plasma supplies an energy flux ∼3 × 10{sup 5} erg cm{sup −2} s{sup −1} = 3 × 10{sup 2} J m{sup −2} s{sup −1} into the corona. The CMR–DL mechanism introduces sudden transitions in the TR as microstructures in both density and energy. The global transition in the TR could be a fractal structure containing such microscopic features. If not impossible, it is difficult to measure such microstructures, but it seems that the coronal heating begins in the nearly collisionless TR by CMR and DL formation.« less

New numerical approaches for modeling thermochemical convection in a compositionally stratified fluid

NASA Astrophysics Data System (ADS)

Puckett, Elbridge Gerry; Turcotte, Donald L.; He, Ying; Lokavarapu, Harsha; Robey, Jonathan M.; Kellogg, Louise H.

2018-03-01

Geochemical observations of mantle-derived rocks favor a nearly homogeneous upper mantle, the source of mid-ocean ridge basalts (MORB), and heterogeneous lower mantle regions. Plumes that generate ocean island basalts are thought to sample the lower mantle regions and exhibit more heterogeneity than MORB. These regions have been associated with lower mantle structures known as large low shear velocity provinces (LLSVPS) below Africa and the South Pacific. The isolation of these regions is attributed to compositional differences and density stratification that, consequently, have been the subject of computational and laboratory modeling designed to determine the parameter regime in which layering is stable and understanding how layering evolves. Mathematical models of persistent compositional interfaces in the Earth's mantle may be inherently unstable, at least in some regions of the parameter space relevant to the mantle. Computing approximations to solutions of such problems presents severe challenges, even to state-of-the-art numerical methods. Some numerical algorithms for modeling the interface between distinct compositions smear the interface at the boundary between compositions, such as methods that add numerical diffusion or 'artificial viscosity' in order to stabilize the algorithm. We present two new algorithms for maintaining high-resolution and sharp computational boundaries in computations of these types of problems: a discontinuous Galerkin method with a bound preserving limiter and a Volume-of-Fluid interface tracking algorithm. We compare these new methods with two approaches widely used for modeling the advection of two distinct thermally driven compositional fields in mantle convection computations: a high-order accurate finite element advection algorithm with entropy viscosity and a particle method that carries a scalar quantity representing the location of each compositional field. All four algorithms are implemented in the open source finite element code ASPECT, which we use to compute the velocity, pressure, and temperature associated with the underlying flow field. We compare the performance of these four algorithms on three problems, including computing an approximation to the solution of an initially compositionally stratified fluid at Ra =105 with buoyancy numbers B that vary from no stratification at B = 0 to stratified flow at large B .

Building a Better Model: A Personalized Breast Cancer Risk Model Incorporating Breast Density to Stratify Risk and Improve Application of Resources

DTIC Science & Technology

2014-10-01

density using automated methods will be optimized during this study through the evaluation of outlier correction, comparison of several different...7 VBD comparison y = 1.3477x - 1.3764 R2 = 0.8213 0 10 20 30 40 50 60 70 0 10 20 30 40 50 VBD volpara [%] VB D cu m ul us [% ] VBD cumulus...Access database and chart review. 5c. Conduct chart review for selected cases (month 4-6). Comparison of information from the Breast Cancer

Data-Driven Asthma Endotypes Defined from Blood Biomarker and Gene Expression Data

PubMed Central

George, Barbara Jane; Reif, David M.; Gallagher, Jane E.; Williams-DeVane, ClarLynda R.; Heidenfelder, Brooke L.; Hudgens, Edward E.; Jones, Wendell; Neas, Lucas; Hubal, Elaine A. Cohen; Edwards, Stephen W.

2015-01-01

The diagnosis and treatment of childhood asthma is complicated by its mechanistically distinct subtypes (endotypes) driven by genetic susceptibility and modulating environmental factors. Clinical biomarkers and blood gene expression were collected from a stratified, cross-sectional study of asthmatic and non-asthmatic children from Detroit, MI. This study describes four distinct asthma endotypes identified via a purely data-driven method. Our method was specifically designed to integrate blood gene expression and clinical biomarkers in a way that provides new mechanistic insights regarding the different asthma endotypes. For example, we describe metabolic syndrome-induced systemic inflammation as an associated factor in three of the four asthma endotypes. Context provided by the clinical biomarker data was essential in interpreting gene expression patterns and identifying putative endotypes, which emphasizes the importance of integrated approaches when studying complex disease etiologies. These synthesized patterns of gene expression and clinical markers from our research may lead to development of novel serum-based biomarker panels. PMID:25643280

The Predictive Effects of Early Pregnancy Lipid Profiles and Fasting Glucose on the Risk of Gestational Diabetes Mellitus Stratified by Body Mass Index.

PubMed

Wang, Chen; Zhu, Weiwei; Wei, Yumei; Su, Rina; Feng, Hui; Lin, Li; Yang, Huixia

2016-01-01

This study aimed at evaluating the predictive effects of early pregnancy lipid profiles and fasting glucose on the risk of gestational diabetes mellitus (GDM) in patients stratified by prepregnancy body mass index (p-BMI) and to determine the optimal cut-off values of each indicator for different p-BMI ranges. A retrospective system cluster sampling survey was conducted in Beijing during 2013 and a total of 5,265 singleton pregnancies without prepregnancy diabetes were included. The information for each participant was collected individually using questionnaires and medical records. Logistic regression analysis and receiver operator characteristics analysis were used in the analysis. Outcomes showed that potential markers for the prediction of GDM include early pregnancy lipid profiles (cholesterol, triacylglycerols, low-density lipoprotein cholesterol/high-density lipoprotein cholesterol ratios [LDL-C/HDL-C], and triglyceride to high-density lipoprotein cholesterol ratios [TG/HDL-C]) and fasting glucose, of which fasting glucose level was the most accurate indicator. Furthermore, the predictive effects and cut-off values for these factors varied according to p-BMI. Thus, p-BMI should be a consideration for the risk assessment of pregnant patients for GDM development.

Effects of Fishing and Fishing Closures on Beach Clams: Experimental Evaluation across Commercially Fished and Non-Fished Beaches before and during Harvesting

PubMed Central

Gray, Charles A.

2016-01-01

Management responses to reconcile declining fisheries typically include closed areas and times to fishing. This study evaluated this strategy for a beach clam fishery by testing the hypothesis that changes in the densities and size compositions of clams from before to during harvesting would differ between commercially fished and non-fished beaches. Sampling was spatially stratified across the swash and dry sand habitats on each of two commercially fished and two non-fished beaches, and temporally stratified across three six-week blocks: before, early and late harvesting. Small-scale spatio-temporal variability in the densities and sizes of clams was prevalent across both habitats and the components of variation were generally greatest at the lowest levels examined. Despite this, differences in the densities and sizes of clams among individual beaches were evident, but there were few significant differences across the commercially fished versus non-fished beaches from before to during harvesting. There was no evidence of reduced densities or truncated size compositions of clams on fished compared to non-fished beaches, contrasting reports of some other organisms in protected areas. This was probably due to a combination of factors, including the current levels of commercial harvests, the movements and other local-scale responses of clams to ecological processes acting independently across individual beaches. The results identify the difficulties in detecting fishing-related impacts against inherent levels of variability in clam populations. Nevertheless, continued experimental studies that test alternate management arrangements may help refine and determine the most suitable strategies for the sustainable harvesting of beach clams, ultimately enhancing the management of sandy beaches. PMID:26731102

Effects of Fishing and Fishing Closures on Beach Clams: Experimental Evaluation across Commercially Fished and Non-Fished Beaches before and during Harvesting.

PubMed

Gray, Charles A

2016-01-01

Management responses to reconcile declining fisheries typically include closed areas and times to fishing. This study evaluated this strategy for a beach clam fishery by testing the hypothesis that changes in the densities and size compositions of clams from before to during harvesting would differ between commercially fished and non-fished beaches. Sampling was spatially stratified across the swash and dry sand habitats on each of two commercially fished and two non-fished beaches, and temporally stratified across three six-week blocks: before, early and late harvesting. Small-scale spatio-temporal variability in the densities and sizes of clams was prevalent across both habitats and the components of variation were generally greatest at the lowest levels examined. Despite this, differences in the densities and sizes of clams among individual beaches were evident, but there were few significant differences across the commercially fished versus non-fished beaches from before to during harvesting. There was no evidence of reduced densities or truncated size compositions of clams on fished compared to non-fished beaches, contrasting reports of some other organisms in protected areas. This was probably due to a combination of factors, including the current levels of commercial harvests, the movements and other local-scale responses of clams to ecological processes acting independently across individual beaches. The results identify the difficulties in detecting fishing-related impacts against inherent levels of variability in clam populations. Nevertheless, continued experimental studies that test alternate management arrangements may help refine and determine the most suitable strategies for the sustainable harvesting of beach clams, ultimately enhancing the management of sandy beaches.

Bone mineral density and mammographic density in Mexican women.

PubMed

Moseson, Heidi; Rice, Megan S; López-Ridaura, Ruy; Bertrand, Kimberly A; Torres, Gabriela; Blanco, Margarita; Tamayo-Orozco, Juan Alfredo; Lajous, Martin; Romieu, Isabelle

2016-01-01

Bone mineral density (BMD) is a putative marker for lifetime exposure to estrogen. Studies that have explored whether BMD is a determinant of mammographic density (MD) have observed inconsistent results. Therefore,we examined this potential association in a sample of women (n = 1,516) from the clinical sub-cohort in the Mexican teachers’ cohort (n = 115,315). We used multivariable linear regression to assess the association between quartiles of BMD and percent MD, as well as total dense and non-dense area of the breast, stratified by menopausal status. We also examined the associations by body mass index (BMI) (< 30 kg/m(2), ≥ 30 kg/m(2)). Overall, there was no association between BMD and MD among premenopausal women. However, when we stratified by BMI, there was a modest inverse association between BMD and percent MD (difference between extreme quartiles = -2.8, 95 % CI -5.9, 0.27, p trend = 0.04) among women with BMI < 30 kg/m(2), but a positive association among obese women (comparable difference = 5.1, 95 % CI 0.02, 10.1, p trend = 0.03;p interaction < 0.01). Among postmenopausal women, BMD and percent MD were positively associated after adjustment for BMI (p trend < 0.01). Postmenopausal women in the highest two quartiles of BMD had 4–5 % point higher percent MD compared to women in the lowest quartile. The association did not differ by BMI in postmenopausal women (p interaction = 0.76). Among obese premenopausal women as well as postmenopausal women, BMD was positively associated with percent MD. Among leaner premenopausal women, BMD and percent MD were modestly inversely associated. These findings support the hypothesis that cumulative exposure to estrogen (as measured by BMD) may influence MD.

Bone mineral density and mammographic density in Mexican women

PubMed Central

Moseson, Heidi; Rice, Megan S.; López-Ridaura, Ruy; Bertrand, Kimberly A.; Torres, Gabriela; Blanco, Margarita; Tamayo-Orozco, Juan Alfredo; Lajous, Martin; Romieu, Isabelle

2016-01-01

Background Bone mineral density (BMD) is a putative marker for lifetime exposure to estrogen. Studies that have explored whether BMD is a determinant of mammographic density (MD) have observed inconsistent results. Therefore, we examined this potential association in a sample of women (N=1,516) from the clinical sub-cohort in the Mexican Teachers’ Cohort (N=115,315). Methods We used multivariable linear regression to assess the association between quartiles of BMD and percent MD, as well as total dense and non-dense area of the breast, stratified by menopausal status. We also examined the associations by body mass index (BMI) (<30kg/m2,, ≥30kg/m2). Results Overall, there was no association between BMD and MD among premenopausal women. However, when we stratified by BMI, there was a modest inverse association between BMD and percent MD (difference between extreme quartiles= −2.8, 95%CI: −5.9, 0.27, p-trend=0.04) among women with BMI <30 kg/m2, but a positive association among obese women (comparable difference=5.1, 95%CI: 0.02, 10.1, p-trend=0.03; p-interaction<0.01). Among postmenopausal women, BMD and percent MD were positively associated after adjustment for BMI (p-trend<0.01). Postmenopausal women in the highest two quartiles of BMD had 4–5 percentage point higher percent MD compared to women in the lowest quartile. The association did not differ by BMI in postmenopausal women (p-interaction=0.76). Conclusion Among obese premenopausal women as well as postmenopausal women, BMD was positively associated with percent MD. Among leaner premenopausal women, BMD and percent MD were modestly inversely associated. These findings support the hypothesis that cumulative exposure to estrogen (as measured by BMD) may influence MD. PMID:26463740

Magnetic flux concentrations from turbulent stratified convection

NASA Astrophysics Data System (ADS)

Käpylä, P. J.; Brandenburg, A.; Kleeorin, N.; Käpylä, M. J.; Rogachevskii, I.

2016-04-01

Context. The formation of magnetic flux concentrations within the solar convection zone leading to sunspot formation is unexplained. Aims: We study the self-organization of initially uniform sub-equipartition magnetic fields by highly stratified turbulent convection. Methods: We perform simulations of magnetoconvection in Cartesian domains representing the uppermost 8.5-24 Mm of the solar convection zone with the horizontal size of the domain varying between 34 and 96 Mm. The density contrast in the 24 Mm deep models is more than 3 × 103 or eight density scale heights, corresponding to a little over 12 pressure scale heights. We impose either a vertical or a horizontal uniform magnetic field in a convection-driven turbulent flow in set-ups where no small-scale dynamos are present. In the most highly stratified cases we employ the reduced sound speed method to relax the time step constraint arising from the high sound speed in the deep layers. We model radiation via the diffusion approximation and neglect detailed radiative transfer in order to concentrate on purely magnetohydrodynamic effects. Results: We find that super-equipartition magnetic flux concentrations are formed near the surface in cases with moderate and high density stratification, corresponding to domain depths of 12.5 and 24 Mm. The size of the concentrations increases as the box size increases and the largest structures (20 Mm horizontally near the surface) are obtained in the models that are 24 Mm deep. The field strength in the concentrations is in the range of 3-5 kG, almost independent of the magnitude of the imposed field. The amplitude of the concentrations grows approximately linearly in time. The effective magnetic pressure measured in the simulations is positive near the surface and negative in the bulk of the convection zone. Its derivative with respect to the mean magnetic field, however, is positive in most of the domain, which is unfavourable for the operation of the negative effective magnetic pressure instability (NEMPI). Simulations in which a passive vector field is evolved do not show a noticeable difference from magnetohydrodynamic runs in terms of the growth of the structures. Furthermore, we find that magnetic flux is concentrated in regions of converging flow corresponding to large-scale supergranulation convection pattern. Conclusions: The linear growth of large-scale flux concentrations implies that their dominant formation process is a tangling of the large-scale field rather than an instability. One plausible mechanism that can explain both the linear growth and the concentration of the flux in the regions of converging flow pattern is flux expulsion. A possible reason for the absence of NEMPI is that the derivative of the effective magnetic pressure with respect to the mean magnetic field has an unfavourable sign. Furthermore, there may not be sufficient scale separation, which is required for NEMPI to work. Movies associated to Figs. 4 and 5 are available in electronic form at http://www.aanda.org

The effect of existing turbulence on stratified shear instability

NASA Astrophysics Data System (ADS)

Kaminski, Alexis; Smyth, William

2017-11-01

Ocean turbulence is an essential process governing, for example, heat uptake by the ocean. In the stably-stratified ocean interior, this turbulence occurs in discrete events driven by vertical variations of the horizontal velocity. Typically, these events have been modelled by assuming an initially laminar stratified shear flow which develops wavelike instabilities, becomes fully turbulent, and then relaminarizes into a stable state. However, in the real ocean there is always some level of turbulence left over from previous events, and it is not yet understood how this turbulence impacts the evolution of future mixing events. Here, we perform a series of direct numerical simulations of turbulent events developing in stratified shear flows that are already at least weakly turbulent. We do so by varying the amplitude of the initial perturbations, and examine the subsequent development of the instability and the impact on the resulting turbulent fluxes. This work is supported by NSF Grant OCE1537173.

Characterization and modeling of turbidity density plume induced into stratified reservoir by flood runoffs.

PubMed

Chung, S W; Lee, H S

2009-01-01

In monsoon climate area, turbidity flows typically induced by flood runoffs cause numerous environmental impacts such as impairment of fish habitat and river attraction, and degradation of water supply efficiency. This study was aimed to characterize the physical dynamics of turbidity plume induced into a stratified reservoir using field monitoring and numerical simulations, and to assess the effect of different withdrawal scenarios on the control of downstream water quality. Three different turbidity models (RUN1, RUN2, RUN3) were developed based on a two-dimensional laterally averaged hydrodynamic and transport model, and validated against field data. RUN1 assumed constant settling velocity of suspended sediment, while RUN2 estimated the settling velocity as a function of particle size, density, and water temperature to consider vertical stratification. RUN3 included a lumped first-order turbidity attenuation rate taking into account the effects of particles aggregation and degradable organic particles. RUN3 showed best performance in replicating the observed variations of in-reservoir and release turbidity. Numerical experiments implemented to assess the effectiveness of different withdrawal depths showed that the alterations of withdrawal depth can modify the pathway and flow regimes of the turbidity plume, but its effect on the control of release water quality could be trivial.

The effects of stand structure after thinning on the growth of an Allegheny hardwood stand

Treesearch

David A. Marquis; Richard L. Ernst

1991-01-01

A 50-year-old Allegheny hardwood stand in which the crown canopy had stratified into distinct species groups was thinned to 60% relative density leaving dramatically different stand structures and species composition. Treatments included combined thinning, thin from middle, thin from above, thin from below, and unthinned control. Individual tree growth was stimulated...

Global Characteristics of Porosity and Density Stratification Within the Lunar Crust from GRAIL Gravity and Lunar Orbiter Laser Altimeter Topography Data

NASA Technical Reports Server (NTRS)

Han, Shin-Chan; Schmerr, Nicholas; Neumann, Gregory; Holmes, Simon

2014-01-01

The Gravity Recovery and Interior Laboratory (GRAIL) mission is providing unprecedentedly high-resolution gravity data. The gravity signal in relation to topography decreases from 100 km to 30 km wavelength, equivalent to a uniform crustal density of 2450 kg/cu m that is 100 kg/cu m smaller than the density required at 100 km. To explain such frequency-dependent behavior, we introduce rock compaction models under lithostatic pressure that yield radially stratified porosity (and thus density) and examine the depth extent of porosity. Our modeling and analysis support the assertion that the crustal density must vary from surface to deep crust by up to 500 kg/cu m. We found that the surface density of mega regolith is around 2400 kg/cu m with an initial porosity of 10-20%, and this porosity is eliminated at 10-20 km depth due to lithostatic overburden pressure. Our stratified density models provide improved fits to both GRAIL primary and extended mission data.

Revising the `Henry Problem' of density-driven groundwater flow: A review of historic Biscayne aquifer data.

NASA Astrophysics Data System (ADS)

Weyer, K. U.

2016-12-01

Coastal groundwater flow investigations at the Cutler site of the Biscayne Bay south of Miami, Florida, gave rise to the dominating concept of density-driven flow of sea water into coastal aquifers indicated as a saltwater wedge. Within that wedge convection type return flow of seawater and a dispersion zone were concluded by Cooper et al. (1964, USGS Water Supply Paper 1613-C) to be the cause of the Biscayne aquifer `sea water wedge'. This conclusion was merely based on the chloride distribution within the aquifer and on an analytical model concept assuming convection flow within a confined aquifer without taking non-chemical field data into consideration. This concept was later labelled the `Henry Problem', which any numerical variable density flow program has to be able to simulate to be considered acceptable. Revisiting the above summarizing publication with its record of piezometric field data (heads) showed that the so-called sea water wedge was actually caused by discharging deep saline groundwater driven by gravitational flow and not by denser sea water. Density driven flow of seawater into the aquifer was not found reflected in the head measurements for low and high tide conditions which had been taken contemporaneously with the chloride measurements. These head measurements had not been included in the flow interpretation. The very same head measurements indicated a clear dividing line between shallow local fresh groundwater flow and saline deep groundwater flow without the existence of a dispersion zone or a convection cell. The Biscayne situation emphasizes the need for any chemical interpretation of flow pattern to be backed up by head data as energy indicators of flow fields. At the Biscayne site density driven flow of seawater did and does not exist. Instead this site and the Florida coast line in general are the end points of local fresh and regional saline groundwater flow systems driven by gravity forces and not by density differences.

Spatiotemporal variation of Van der Burgh's coefficient in a salt plug estuary

NASA Astrophysics Data System (ADS)

Shaha, Dinesh Chandra; Cho, Yang-Ki; Kim, Bong Guk; Rafi Afruz Sony, M.; Rani Kundu, Sampa; Faruqul Islam, M.

2017-09-01

Salt water intrusion in estuaries is expected to become a serious global issue due to climate change. Van der Burgh's coefficient, K, is a good proxy for describing the relative contribution of tide-driven and gravitational (discharge-driven and density-driven) components of salt transport in estuaries. However, debate continues over the use of the K value for an estuary where K should be a constant, spatially varying, or time-independent factor for different river discharge conditions. In this study, we determined K during spring and neap tides in the dry (< 30 m-3 s-1) and wet (> 750 m-3 s-1) seasons in a salt plug estuary with an exponentially varying width and depth, to examine the relative contributions of tidal versus density-driven salt transport mechanisms. High-resolution salinity data were used to determine K. Discharge-driven gravitational circulation (K ˜ 0.8) was entirely dominant over tidal dispersion during spring and neap tides in the wet season, to the extent that salt transport upstream was effectively reduced, resulting in the estuary remaining in a relatively fresh state. In contrast, K increased gradually seaward (K ˜ 0.74) and landward (K ˜ 0.74) from the salt plug area (K ˜ 0.65) during the dry season, similar to an inverse and positive estuary, respectively. As a result, density-driven inverse gravitational circulation between the salt plug and the sea facilitates inverse estuarine circulation. On the other hand, positive estuarine circulation between the salt plug and the river arose due to density-driven positive gravitational circulation during the dry season, causing the upstream intrusion of high-salinity bottom water. Our results explicitly show that K varies spatially and depends on the river discharge. This result provides a better understanding of the distribution of hydrographic properties.

Understanding exchanges across turbulent/stratified zones interfaces

NASA Astrophysics Data System (ADS)

Le Bars, M.; Ribeiro, A.; Le Gal, P.; Aurnou, J. M.

2013-12-01

In many geophysical and astrophysical situations, a turbulent fluid layer is separated from a stably stratified one by a relatively sharp but deformable interface. Examples include the convective and radiative zones in stars, the atmospheric convective layer and overlying stratosphere, the Earth's outer core... While motions in the stratified layer are often neglected, it actually supports oscillatory motions called gravito-inertial waves (GIW) excited by Reynolds stresses, entropy fluctuations and interface deformations associated with the turbulence. Besides their direct observation as for instance in asteroseismology, GIW transport energy, carry momentum, break, mix... and are thus essential for accurate models of global climate and solar or core dynamics. Global integrated models including length scales and time scales spanning many orders of magnitude are required to fully address motions in turbulent and stratified zones and to understand the details of the highly non-linear couplings between rotation, meridional circulation, turbulence and waves: this is clearly very challenging from both analytical and numerical points of view. Here, we present results from two complementary laboratory experiments using water as a working fluid and salt or temperature to control the buoyancy effects, allowing to address the whole range of relevant physical issues in simplified models. In the first set-up, we take benefit from the unusual property of water that its density has a maximum value near 4oC to study its convective and oscillatory motions in a tank with a bottom boundary at about 0oC and a hotter upper surface. High precision local measurements of temperature fluctuations are performed simultaneously in the convective and stratified zones to produce the corresponding power density spectrum and probability density function. In the second set-up, a turbulent jet generated by injection of water impinges upon the interface between a uniform density layer and a stratified one of salted water. The experiment is performed on a rotating table. Velocity measurements are carried out non-intrusively using Particle Imaging Velocimetry in both regions. Our combined results show that (i) the interface acts as a filter which mostly allows for the passing of low frequency perturbations, while (ii) the further propagation of the excited waves in the stratified zone gives rise to selective wave damping, focusing the transported energy and momentum around given frequencies corresponding to selected propagation angles. We explain those results with a simple analytical model, allowing their extension to natural configurations.

Stratified flows with variable density: mathematical modelling and numerical challenges.

NASA Astrophysics Data System (ADS)

Murillo, Javier; Navas-Montilla, Adrian

2017-04-01

Stratified flows appear in a wide variety of fundamental problems in hydrological and geophysical sciences. They may involve from hyperconcentrated floods carrying sediment causing collapse, landslides and debris flows, to suspended material in turbidity currents where turbulence is a key process. Also, in stratified flows variable horizontal density is present. Depending on the case, density varies according to the volumetric concentration of different components or species that can represent transported or suspended materials or soluble substances. Multilayer approaches based on the shallow water equations provide suitable models but are not free from difficulties when moving to the numerical resolution of the governing equations. Considering the variety of temporal and spatial scales, transfer of mass and energy among layers may strongly differ from one case to another. As a consequence, in order to provide accurate solutions, very high order methods of proved quality are demanded. Under these complex scenarios it is necessary to observe that the numerical solution provides the expected order of accuracy but also converges to the physically based solution, which is not an easy task. To this purpose, this work will focus in the use of Energy balanced augmented solvers, in particular, the Augmented Roe Flux ADER scheme. References: J. Murillo , P. García-Navarro, Wave Riemann description of friction terms in unsteady shallow flows: Application to water and mud/debris floods. J. Comput. Phys. 231 (2012) 1963-2001. J. Murillo B. Latorre, P. García-Navarro. A Riemann solver for unsteady computation of 2D shallow flows with variable density. J. Comput. Phys.231 (2012) 4775-4807. A. Navas-Montilla, J. Murillo, Energy balanced numerical schemes with very high order. The Augmented Roe Flux ADER scheme. Application to the shallow water equations, J. Comput. Phys. 290 (2015) 188-218. A. Navas-Montilla, J. Murillo, Asymptotically and exactly energy balanced augmented flux-ADER schemes with application to hyperbolic conservation laws with geometric source terms, J. Comput. Phys. 317 (2016) 108-147. J. Murillo and A. Navas-Montilla, A comprehensive explanation and exercise of the source terms in hyperbolic systems using Roe type solutions. Application to the 1D-2D shallow water equations, Advances in Water Resources 98 (2016) 70-96.

Multielectron effects in the photoelectron momentum distribution of noble-gas atoms driven by visible-to-infrared-frequency laser pulses: A time-dependent density-functional-theory approach

NASA Astrophysics Data System (ADS)

Murakami, Mitsuko; Zhang, G. P.; Chu, Shih-I.

2017-05-01

We present the photoelectron momentum distributions (PMDs) of helium, neon, and argon atoms driven by a linearly polarized, visible (527-nm) or near-infrared (800-nm) laser pulse (20 optical cycles in duration) based on the time-dependent density-functional theory (TDDFT) under the local-density approximation with a self-interaction correction. A set of time-dependent Kohn-Sham equations for all electrons in an atom is numerically solved using the generalized pseudospectral method. An effect of the electron-electron interaction driven by a visible laser field is not recognizable in the helium and neon PMDs except for a reduction of the overall photoelectron yield, but there is a clear difference between the PMDs of an argon atom calculated with the frozen-core approximation and TDDFT, indicating an interference of its M -shell wave functions during the ionization. Furthermore, we find that the PMDs of degenerate p states are well separated in intensity when driven by a near-infrared laser field, so that the single-active-electron approximation can be adopted safely.

Calculation of periodic flows in a continuously stratified fluid

NASA Astrophysics Data System (ADS)

Vasiliev, A.

2012-04-01

Analytic theory of disturbances generated by an oscillating compact source in a viscous continuously stratified fluid was constructed. Exact solution of the internal waves generation problem was constructed taking into account diffusivity effects. This analysis is based on set of fundamental equations of incompressible flows. The linearized problem of periodic flows in a continuously stratified fluid, generated by an oscillating part of the inclined plane was solved by methods of singular perturbation theory. A rectangular or disc placed on a sloping plane and oscillating linearly in an arbitrary direction was selected as a source of disturbances. The solutions include regularly perturbed on dissipative component functions describing internal waves and a family of singularly perturbed functions. One of the functions from the singular components family has an analogue in a homogeneous fluid that is a periodic or Stokes' flow. Its thickness is defined by a universal micro scale depending on kinematics viscosity coefficient and a buoyancy frequency with a factor depending on the wave slope. Other singular perturbed functions are specific for stratified flows. Their thickness are defined the diffusion coefficient, kinematic viscosity and additional factor depending on geometry of the problem. Fields of fluid density, velocity, vorticity, pressure, energy density and flux as well as forces acting on the source are calculated for different types of the sources. It is shown that most effective source of waves is the bi-piston. Complete 3D problem is transformed in various limiting cases that are into 2D problem for source in stratified or homogeneous fluid and the Stokes problem for an oscillating infinite plane. The case of the "critical" angle that is equality of the emitting surface and the wave cone slope angles needs in separate investigations. In this case, the number of singular component is saved. Patterns of velocity and density fields were constructed and analyzed by methods of computational mathematics. Singular components of the solution affect the flow pattern of the inhomogeneous stratified fluid, not only near the source of the waves, but at a large distance. Analytical calculations of the structure of wave beams are matched with laboratory experiments. Some deviations at large distances from the source are formed due to the contribution of background wave field associated with seiches in the laboratory tank. In number of the experiments vortices with closed contours were observed on some distances from the disk. The work was supported by Ministry of Education and Science RF (Goscontract No. 16.518.11.7059), experiments were performed on set up USU "HPC IPMec RAS".

Spatial and Temporal Trends in the Density Stratification of Long Island Sound

NASA Astrophysics Data System (ADS)

Marchese, P.

2017-12-01

The density structure of Long Island Sound (LIS) was studied using historical hydrographic data. Like many estuaries, LIS suffers from hypoxia during the summer months; a result of the density stratification caused by surface warming and weak wind conditions. In summer, the water column is stratified at both ends (east and west) with a vertically well mixed region near the middle. During these months, the western side of LIS experiences low bottom dissolved from the higher nutrient influx and the resulting oxygen demand. Eastern LIS does not experience hypoxia despite sometimes being more highly stratified than the west because these bottom water are regularly ventilated by incoming higher DO water from outside. Topography and density gradients prevent the low DO water from encroaching to the eastern basin. In the fall, changing atmospheric conditions weakens the density stratification throughout LIS, although in some regions the vertical gradient will persist, sometimes until January.

EXPERIMENTS ON STABLY AND NEUTRALLY STRATIFIED FLOW OVER A MODEL THREE-DIMENSIONAL HILL

EPA Science Inventory

The flow structure over a bell shaped hill (reciprocal of a fourth order polynomial in cross section and height h) was studied in large and small stably stratified towing tanks (with uniform density gradients) and in an unstratified wind tunnel. Observations were made at Froude n...

pi-eta mixing and charge symmetry violating NN potential in matter

DOE Office of Scientific and Technical Information (OSTI.GOV)

Biswas, Subhrajyoti; Roy, Pradip; Dutt-Mazumder, Abhee K.

2010-06-15

We construct density-dependent class III charge symmetry violating (CSV) potential caused by the mixing of pi-eta mesons with off-shell corrections. The density dependence enters through the nonvanishing pi-eta mixing driven by both the neutron-proton mass difference and their asymmetric density distribution. The contribution of density-dependent mixing to the CSV potential is found to be appreciably larger than that of the vacuum part.

On the Single-Layer Hydraulics Model for Flows and Ventilation over Unban Areas in Stable Stratification

NASA Astrophysics Data System (ADS)

Liu, C. H.

2015-12-01

Atmospheric stability has substantial effects on the flows and heat/mass transport processes. While extensive studies have been conducted for neutral and unstable stabilities, rather limited studies have been devoted to stable stratification. Major technical reason is the demanding spatio-temporal resolution required to solve the small scales in stratified turbulent flows. Instead of continuous density variation, we use the single-layer hydraulics model (analogous to shallow water equations for global dynamics), to simulate the stratified flows and turbulence structure over hypothetical urban areas. An array of identical ribs in cross flows is used to model an idealized urban surface and the aerodynamic resistance is controlled by the separation among the ribs. Two immiscible fluids (water and air) with a large density difference (three order of magnitude) are used to simulate the stratification. The key assumption is that the density in the (lower) single layer is uniform. As a result, the stratification is measured by the Froude number Fr (= U/(gH)1/2; where U is the flow speed, g the gravitational acceleration and H the single-layer depth). One of the characteristics of single-layer hydraulics model is hydraulic jump which occurs when the flows are slowing down from Fr > 1 (high-speed flows over smoother surfaces) to Fr < 1 (lower-speed flows over rougher surfaces). It is noteworthy that kinetic energy does not conserve across hydraulic jump that, unavoidably, cascades to turbulent kinetic energy (TKE). We thus hypotheses that the elevated TKE could modify the street-level ventilation mechanism in the stratified flows across an abrupt change in surface roughness entering urban areas. Large-eddy simulation and laboratory-scale water channel experiments are sought to improve our understanding of the occurrence of hydraulic jump and the associated street-level ventilation mechanism in the stratified flows over urban areas. Preliminary results, by comparing the dynamics at Fr = 2.4 and Fr = 2.8, demonstrate the notable changes in ventilation performance in the first several rows of ribs of urban areas. Substantial changes in the mean and fluctuating velocities are observed that contribute to the different street-level ventilation mechanism. Detailed results will be reported in the upcoming AGU fall meeting.

A Theory of Density Layering in Stratified Turbulence using Statistical State Dynamics

NASA Astrophysics Data System (ADS)

Fitzgerald, J.; Farrell, B.

2016-12-01

Stably stratified turbulent fluids commonly develop density structures that are layered in the vertical direction (e.g., Manucharyan et al., 2015). Within layers, density is approximately constant and stratification is weak. Between layers, density varies rapidly and stratification is strong. A common explanation for the existence of layers invokes the negative diffusion mechanism of Phillips (1972) & Posmentier (1977). The physical principle underlying this mechanism is that the flux-gradient relationship connecting the turbulent fluxes of buoyancy to the background stratification must have the special property of weakening fluxes with strengthening gradient. Under these conditions, the evolution of the stratification is governed by a negative diffusion problem which gives rise to spontaneous layer formation. In previous work on stratified layering, this flux-gradient property is often assumed (e.g, Posmentier, 1977) or drawn from phenomenological models of turbulence (e.g., Balmforth et al., 1998).In this work we develop the theoretical underpinnings of layer formation by applying stochastic turbulence modeling and statistical state dynamics (SSD) to predict the flux-gradient relation and analyze layer formation directly from the equations of motion. We show that for stochastically-forced homogeneous 2D Boussinesq turbulence, the flux-gradient relation can be obtained analytically and indicates that the fluxes always strengthen with stratification. The Phillips mechanism thus does not operate in this maximally simplified scenario. However, when the problem is augmented to include a large scale background shear, we show that the flux-gradient relationship is modified so that the fluxes weaken with stratification. Sheared and stratified 2D Boussinesq turbulence thus spontaneously forms density layers through the Phillips mechanism. Using SSD (Farrell & Ioannou 2003), we obtain a closed, deterministic dynamics for the stratification and the statistical turbulent state. We show that density layers form as a linear instability of the sheared turbulence, associated with a supercritical bifurcation. We further show that SSD predicts the nonlinear equilibration and maintenance of the layers, and captures the phenomena of layer growth and mergers (Radko, 2007).

Structure-driven turbulence in ``No man's Land''

NASA Astrophysics Data System (ADS)

Kosuga, Yusuke; Diamond, Patrick

2012-10-01

Structures are often observed in many physical systems. In tokamaks, for example, such structures are observed as density blobs and holes. Such density blobs and holes are generated at the tokamak edge, where strong gradient perturbations generate an outgoing blob and an incoming hole. Since density holes can propagate from the edge to the core, such structures may play an important role in understanding the phenomenology of the edge-core coupling region, so-called ``No Man's Land.'' In this work, we discuss the dynamics of such structures in real space. In particular, we consider the dynamics of density blobs and holes in the Hasegawa-Wakatani system. Specific questions addressed here include: i) how these structures extract free energy and enhance transport? how different is the relaxation driven by such structures from that driven by linear drift waves? ii) how these structures interact with shear flows? In particular, how these structures interact with a shear layer, which can absorb structures resonantly? iii) how can we calculate the coupled evolution of structures and shear flows? Implications for edge-core coupling problem are discussed as well.

Development of a sampling strategy and sample size calculation to estimate the distribution of mammographic breast density in Korean women.

PubMed

Jun, Jae Kwan; Kim, Mi Jin; Choi, Kui Son; Suh, Mina; Jung, Kyu-Won

2012-01-01

Mammographic breast density is a known risk factor for breast cancer. To conduct a survey to estimate the distribution of mammographic breast density in Korean women, appropriate sampling strategies for representative and efficient sampling design were evaluated through simulation. Using the target population from the National Cancer Screening Programme (NCSP) for breast cancer in 2009, we verified the distribution estimate by repeating the simulation 1,000 times using stratified random sampling to investigate the distribution of breast density of 1,340,362 women. According to the simulation results, using a sampling design stratifying the nation into three groups (metropolitan, urban, and rural), with a total sample size of 4,000, we estimated the distribution of breast density in Korean women at a level of 0.01% tolerance. Based on the results of our study, a nationwide survey for estimating the distribution of mammographic breast density among Korean women can be conducted efficiently.

Forest structure and composition in the lower montane rain forest of the Luquillo Mountains, Puerto Rico

Treesearch

Peter L. Weaver

2010-01-01

Six groups of three plots stratified by aspect and topography and varying in elevation were used to sample forest structure and tree species composition within the lower montane rain forest (tabonuco forest) of the Luquillo Experimental Forest (LEF) in Puerto Rico. Stem density, tree height, and total above ground biomass varied by site. Significant differences in...

Causes of strong ocean heating during glacial periods

NASA Astrophysics Data System (ADS)

Zimov, N.; Zimov, S. A.

2013-12-01

During the last deglaciation period, the strongest climate changes occurred across the North Atlantic regions. Analyses of borehole temperatures from the Greenland ice sheet have yielded air temperature change estimates of 25°C over the deglaciation period (Dahl-Jensen et al. 1998). Such huge temperature changes cannot currently be explained in the frames of modern knowledge about climate. We propose that glacial-interglacial cycles are connected with gradual warming of ocean interior waters over the course of glaciations and quick transport of accumulated heat from ocean to the atmosphere during the deglaciation periods. Modern day ocean circulation is dominated by thermal convection with cold waters subsiding in the Northern Atlantic and filling up the ocean interior with cold and heavy water. However during the glaciation thermal circulation stopped and ocean circulation was driven by 'haline pumps' -Red and Mediterranean seas connected with ocean with only narrow but deep straights acts as evaporative basins, separating ocean water into fresh water which returns to the ocean surface (precipitation) and warm but salty, and therefore heavy, water which flows down to the ocean floor. This haline pump is stratifying the ocean, allowing warmer water locate under the colder water and thus stopping thermal convection in the ocean. Additional ocean interior warming is driven by geothermal heat flux and decomposition of organic rain. To test the hypothesis we present simple ocean box model that describes thermohaline circulation in the World Ocean. The first box is the Red and Mediterranean sea, the second is united high-latitude seas, the third is the ocean surface, and the fourth the ocean interior. The volume of these water masses and straight cross-sections are taken to be close to real values. We have accepted that the exchange of water between boxes is proportional to the difference in water density in these boxes, Sun energy inputs to the ocean and sea surface are taken as constant. Energy income to the interior box from the geothermal heat flux is also taken as constant. Even though energy inputs are taken as constants, the model manages to recreate the glacial-interglacial cycles. In the glacial periods only haline circulation takes place, the ocean is strongly stratified, and the interior box accumulates heat, while high-latitudes accumulate ice. 112,000 years after glaciation starts, water density on the ocean bottom becomes equal to the density of water in high-latitude seas, strong thermal convection take place, and the ocean quickly (within 14,600 years) releases the heat. The magnitude and duration of such cycles correspond with magnitudes and durations reconstructed for actual glacial-interglacial cycles. From the proposed mechanism it follows that during the glaciations it is likely that the Arctic Ocean was a big reservoir of isotopically light fresh ice. If in a glacial period, the World Ocean were half filled with warm water from the Red Sea and bioproductivity of the ocean declined because of the slow circulation, then carbon storage within the ocean reservoir would decline by ~2000 Pg (10^15 g) of carbon.

On the rising motion of a drop in stratified fluids

NASA Astrophysics Data System (ADS)

Bayareh, M.; Doostmohammadi, A.; Dabiri, S.; Ardekani, A. M.

2013-10-01

The rising dynamics of a deformable drop in a linearly stratified fluid is numerically obtained using a finite-volume/front-tracking method. Our results show that the drag coefficient of a spherical drop in a stratified fluid enhances as C_{d,s}/C_{d,h}-1˜ Fr_d^{-2.86} for drop Froude numbers in the range of 4 < Frd < 16. The role of the deformability of the drop on the temporal evolution of the motion is investigated along with stratification and inertial effects. We also present the important role of stratification on the transient rising motion of the drop. It is shown that a drop can levitate in the presence of a vertical density gradient. The drop undergoes a fading oscillatory motion around its neutrally buoyant position except for high viscosity ratio drops where the oscillation occurs around a density level lighter than the neutral buoyancy level. In addition, a detailed characterization of the flow signature of a rising drop in a linearly stratified fluid including the buoyancy induced vortices and the resultant buoyant jet is presented.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Zhaoqing; Wang, Taiping

A three-dimensional coastal ocean model with a tidal turbine module was used in this paper to study the effects of tidal energy extraction on temperature and salinity stratification and density driven two-layer estuarine circulation. Numerical experiments with various turbine array configurations were carried out to investigate the changes in tidally mean temperature, salinity and velocity profiles in an idealized stratified estuary that connects to coastal water through a narrow tidal channel. The model was driven by tides, river inflow and sea surface heat flux. To represent the realistic size of commercial tidal farms, model simulations were conducted based on amore » small percentage of the total number of turbines that would generate the maximum extractable energy in the system. Model results indicated that extraction of tidal energy will increase the vertical mixing and decrease the stratification in the estuary. Extraction of tidal energy has stronger impact on the tidally-averaged salinity, temperature and velocity in the surface layer than the bottom. Energy extraction also weakens the two-layer estuarine circulation, especially during neap tides when tidal mixing the weakest and energy extraction is the smallest. Model results also show that energy generation can be much more efficient with higher hub height with relatively small changes in stratification and two-layer estuarine circulation.« less

Impact of convection and resistivity on angular momentum transport in dwarf novae.

NASA Astrophysics Data System (ADS)

Scepi, N.; Lesur, G.; Dubus, G.; Flock, M.

2017-12-01

The eruptive cycles of dwarf novae are thought to be due to a thermal-viscous instability in the accretion disk surrounding the white dwarf. This model has long been known to imply enhanced angular momentum transport in the accretion disk during outburst. This is measured by the stress to pressure ratio α, with α≈ 0.1 required in outburst compared to α≈ 0.01 in quiescence. Such an enhancement in α has recently been observed in simulations of turbulent transport driven by the magneto-rotational instability (MRI) when convection is present, without requiring a net magnetic flux. We independently recover this result by carrying out PLUTO MHD simulations of vertically stratified, radiative, shearing boxes with the thermodynamics and opacities appropriate to dwarf novae. The results are robust against the choice of vertical boundary conditions. In the quiescent state, the disk is only very weakly ionized so, in the second part of our work, we studied the impact of resistive MHD on transport.We find that the MRI-driven transport is quenched (α≈ 0) below the critical density at which the magnetic Reynolds number R_{m}≤ 10^4. This is problematic because the X-ray emission observed in quiescent systems requires ongoing accretion onto the white dwarf.

Subtidal circulation on the Alabama shelf during the Deepwater Horizon oil spill

NASA Astrophysics Data System (ADS)

Dzwonkowski, Brian; Park, Kyeong

2012-03-01

Water column velocity and hydrographic measurements on the inner Alabama shelf are used to examine the flow field and its forcing dynamics during the Deepwater Horizon oil spill disaster in the spring and summer of 2010. Comparison between two sites provides insight into the flow variability and dynamics of a shallow, highly stratified shelf in the presence of complicating geographic and bathymetric features. Seasonal currents reveal a convergent flow with strong, highly sheared offshore flow near a submarine bank just outside of Mobile Bay. At synoptic time scales, the flow is relatively consistent with typical characteristics of wind-driven Ekman coastal circulation. Analysis of the depth-averaged along-shelf momentum balance indicates that both bottom stress and along-shelf pressure gradient act to counter wind stress. As a consequence of the along-shelf pressure gradient and thermal wind shear, flow reversals in the bottom currents can occur during periods of transitional winds. Despite the relatively short distance between the two sites (14 km), significant spatial variability is observed. This spatial variability is argued to be a result of local variations in the bathymetry and density field as the study region encompasses a submarine bank near the mouth of a major freshwater source. Given the physical parameters of the system, along-shelf flow in this region would be expected to separate from the local isobaths, generating a mean offshore flow. The local, highly variable density field is expected to be, in part, responsible for the differences in the vertical variability in the current profiles.

Buoyancy-driven instabilities around miscible A+B→C reaction fronts: a general classification.

PubMed

Trevelyan, P M J; Almarcha, C; De Wit, A

2015-02-01

Upon contact between miscible solutions of reactants A and B along a horizontal interface in the gravity field, various buoyancy-driven instabilities can develop when an A+B→C reaction takes place and the density varies with the concentrations of the various chemicals. To classify the possible convective instability scenarios, we analyze the spatial dependence of the large time asymptotic density profiles as a function of the key parameters of the problem, which are the ratios of diffusion coefficients and of solutal expansion coefficients of species A, B, and C. We find that 62 different density profiles can develop in the reactive problem, whereas only 6 of them can be obtained in the nonreactive one.

Inertial floaters in stratified turbulence

NASA Astrophysics Data System (ADS)

Sozza, A.; De Lillo, F.; Boffetta, G.

2018-01-01

We investigate numerically the dynamics and statistics of inertial particles transported by stratified turbulence, in the case of particle density intermediate in the average density profile of the fluid. Under these conditions, particles tend to form a thin layer around the corresponding fluid isopycnal. The thickness of the resulting layer is determined by a balance between buoyancy (which attracts the particle to the isopycnal) and inertia (which prevents them from following it exactly). By means of extensive numerical simulations, we explore the parameter space of the system and we find that in a range of parameters particles form fractal clusters within the layer.

Generation of large-scale vorticity in rotating stratified turbulence with inhomogeneous helicity: mean-field theory

NASA Astrophysics Data System (ADS)

Kleeorin, N.

2018-06-01

We discuss a mean-field theory of the generation of large-scale vorticity in a rotating density stratified developed turbulence with inhomogeneous kinetic helicity. We show that the large-scale non-uniform flow is produced due to either a combined action of a density stratified rotating turbulence and uniform kinetic helicity or a combined effect of a rotating incompressible turbulence and inhomogeneous kinetic helicity. These effects result in the formation of a large-scale shear, and in turn its interaction with the small-scale turbulence causes an excitation of the large-scale instability (known as a vorticity dynamo) due to a combined effect of the large-scale shear and Reynolds stress-induced generation of the mean vorticity. The latter is due to the effect of large-scale shear on the Reynolds stress. A fast rotation suppresses this large-scale instability.

The Hydrochemical Evolution of Water-Filled Sinkholes at Bitter Lake NWR, Roswell, NM

NASA Astrophysics Data System (ADS)

Premo, E.; Crossey, L. J.

2013-12-01

Bitter Lake National Wildlife Refuge in Roswell, NM houses one of the most ecologically significant wetlands in the US-SW including approximately 52 water-filled sinkholes each supporting a unique biological assemblage, including several endangered and endemic species (e.g., Pecos pupfish and Noel's amphipod, respectively). Forming in the karst landscape adjacent to the Pecos River where the regional dual-aquifer system discharges through a network of springs and seeps, these sinkholes are recharged by saline groundwater that is subject to anthropogenic withdrawals for irrigation and hydrocarbon production and chemically altered by a complex series of evaporation-precipitation reactions after discharge. This study investigates the hydrochemical differences among these sinkholes while considering the evolutionary processes affecting water column structure, geochemical mixing and ecological sustainability. Two major sampling suites, pre- and post-irrigation, yielded waters from 1.0m increments along the water columns of 10 representative sinkholes. Samples were analyzed for major ions, stable isotopes [δ18O, δD ], and dissolved gases; PHREEQc was used to model mineral saturation and speciation. An in-situ mineral precipitation experiment provided growth rate and mineral morphological (SEM) data. Source water is chemically similar to shallow springs found at the Refuge (Sago Spring). Sinkholes exhibit bimodal water column structure (well-mixed or stratified) organized in response to water density (with ~1.035 g/cm3 forming the modal transition threshold). By measuring the density, TDS or conductivity at sinkhole surface it is possible to predict modality of water column structure. Sinkhole waters - regardless of depth or season - fall along a common isotopic evaporation trajectory (δ D = 3.387*δ18O - 19.38), and adopt a Na-Cl chemical endmember facies. Driven primarily by physical sinkhole geometry (e.g., depth and surface area), sinkhole water follows a predictable evolutionary progression from spring-like well-mixed ('young'), to moderately saline well-mixed ('transitional'), to saline and stratified ('old' or 'evolved'), based on the relative volume of water that has entered and subsequently evaporated from the system. Simple geochemical models reveal calcium- and sulfate-bearing minerals (calcite, gypsum) precipitate early in the reaction while halite and magnesium-containing minerals precipitate late, rendering increased Cl- and Mg+ concentrations in fluids subjected to prolonged evaporation. This water is also high in CO2 content and may contain traces of He, suggesting emergent water is a combination of groundwater (dominant) and deeply sourced fluids (minor). Both PO4 and NH4 are present in biologically-significant concentrations in sinkholes with chemically controlled water columns, and photosynthetic bacteria were found to organize at the bottom of the photic zone. High NH4 and CO2 accompanying low O2 dissolved gas values confirm the increased biological control in stratified sinkholes. Resident fish populations are affected by water chemistry which reduces reproductive success or exceed the survivable range of habitable conditions. Results of this study serve as a geochemical baseline survey of Refuge sinkholes and may be used to both aid with biological resource management and predict stratified conditions using measurable proxies.

From current-driven to neoclassically driven tearing modes.

PubMed

Reimerdes, H; Sauter, O; Goodman, T; Pochelon, A

2002-03-11

In the TCV tokamak, the m/n = 2/1 island is observed in low-density discharges with central electron-cyclotron current drive. The evolution of its width has two distinct growth phases, one of which can be linked to a "conventional" tearing mode driven unstable by the current profile and the other to a neoclassical tearing mode driven by a perturbation of the bootstrap current. The TCV results provide the first clear observation of such a destabilization mechanism and reconcile the theory of conventional and neoclassical tearing modes, which differ only in the dominant driving term.

A simulation of dielectrophoresis force actuated liquid lens

NASA Astrophysics Data System (ADS)

Yao, Xiaoyin; Xia, Jun

2009-11-01

Dielectrophoresis (DEP) and electrowetting on dielectric (EWOD) are based on the electrokinetic mechanisms which have great potential in microfluidic manipulation. DEP dominate the movement of particles induced by polarization effects in nonuniform electric field ,while EWOD has become one of the most widely used tools for manipulating tiny amounts of liquids on solid surfaces. Liquid lens driven by EWOD have been well studied and developed. But liquid lens driven by DEP has not been studied adequately. This paper focuses on modeling liquid lens driven by DEP force. A simulation of DEP driven droplet dynamics was performed by coupling of the electrostatic field and the two-phase flow field. Two incompressible and dielectric liquids with different permittivity were chosen in the two-phase flow field. The DEP force density, in direct proportion to gradient of the square of the electric field intensity, was used as a body force density in Navier-Stokes equation. When voltage applied, the liquid with high permittivity flowed to the place where the gradient of the square of the electric field intensity was higher, and thus change the curvature of interface between two immiscible liquid. The differences between DEP and EWOD liquid lens were also presented.

Effects of Mean Flow Profiles on the Instability of a Low-Density Gas Jet Injected into a High-Density Gas

NASA Technical Reports Server (NTRS)

Vedantam, NandaKishore; Parthasarathy, Ramkumar N.

2004-01-01

The effects of the mean velocity profiles on the instability characteristics in the near-injector region of axisymmetric low density gas jets injected vertically upwards into a high-density gas medium were investigated using linear inviscid stability analysis. The flow was assumed to be isothermal and locally parallel. Three velocity profiles, signifying different changes in the mean velocity in the shear layer, were used in the analysis. The effects of the inhomogeneous shear layer and the Froude number (signifying the effects of gravity) on the instability for each set of mean profiles were delineated. At a large Froude number (negligible gravity), a critical density ratio was found for the three profiles at which the jet became absolutely unstable. The critical density ratio for each velocity profile was increased as the Froude number was reduced. A critical Froude number was found for the three sets of profiles, below which the jet was absolutely unstable for all the density ratios less than unity, which demarcated the jet flow into the momentum-driven regime and the buoyancy-driven regime.

Numerical simulation of stratified flows from laboratory experiments to coastal ocean

NASA Astrophysics Data System (ADS)

Fraunie, Philippe

2014-05-01

Numeric modeling of a flow past vertical strip uniformly towing with permanent velocity in horizontal direction in a linearly stratified talk which was based on a finite differences solver adapted to the low Reynolds Navier-Stokes equation with transport equation for salinity (LES simulation [6]) has demonstrated reasonable agreement with data of schlieren visualization, density marker and probe measurements of internal wave fields. Another approach based on two different numerical methods for one specific case of stably stratified incompressible flow was developed, using the compact finite-difference discretizations. The numerical scheme itself follows the principle of semi-discretisation, with high order compact discretisation in space, while the time integration is carried out by the Strong Stability Preserving Runge-Kutta scheme. Results were compared against the reference solution obtained by the AUSM finite volume method [7]. The test case allowed demonstrating the ability of selected numerical methods to represent stably stratified flows over horizontal strip [4] and hill type 2D obstacles [1, 3] with generation of internal waves. From previous LES [4] and RANS [8] realistic simulations code, the ability of research codes to reproduce field observations is discussed. ACKNOWLEDGMENTS This research work was supported by Region Provence Alpes Côte d'Azur - Modtercom project, the Research Plan MSM 6840770010 of the Ministry of education of Czech Republic and the Russian Foundation for Basic Research (grant 12-01-00128). REFERENCES 1. Chashechkin Yu.D., Mitkin V.V. Experimental study of a fine structure of 2D wakes and mixing past an obstacle in a continuously stratified fluid // Dynamics of Atmosphere and Oceans. 2001. V. 34. P. 165-187. 2. Chashechkin, Yu. D. Hydrodynamics of a sphere in a stratified fluid // Fluid Dyn. 1989. V.24(1) P. 1-7. 3. Mitkin V. V., Chashechkin Yu. D. Transformation of hanging discontinuities into vortex systems in a stratified flow behind a cylinder // 2007. Fluid Dyn. V. 42 (1). P. 12-23. 4. Bardakov R. N., Mitkin V. V., Chashechkin Yu. D. Fine structure of a stratified flow near a flat-plate surface // J. Appl. Mech. Tech. Phys. 2007. V. 48(6) P. 840-851. 5. Chashechkin Yu. D., Mitkin V. V. An effect of a lift force on the structure of attached internal waves in a continuously stratified fluid // Dokl. Phys. 2001. V. 46 (6). P. 425-428. 6. Houcine H., Chashechkin Yu.D, Fraunié P., Fernando H.J.S., Gharbi A., Lili T. Numerical modeling of the generation of internal waves by uniform stratified flow over a thin vertical barrier // Int J. Num Methods in Fluids. 2012. V.68(4). P. 451-466. DOI: 10.1002/fld.2513 7. Bodnar T., Benes , Fraunié P., Kozel K.. Application of Compact Finite-Difference Schemes to Simulations of Stably Stratified Fluid Flows. Applied Mathematics and Computation 219 : 3336-3353 2012. doi:10.1016/j.amc.2011.08.058 8. Schaeffer A. Molcard A. Forget P. Fraunié P. Garreau P. Generation mechanisms for mesoscale eddies in the Gulf of Lions: radar observation and modelling. Ocean Dynamics vol 61, 10, pp1587-1609, 2011. DOI.1007/s10236-011-0482-8.

The enigmatic ultra-long run-out of seafloor density driven flows

NASA Astrophysics Data System (ADS)

Dorrell, R. M.

2017-12-01

Dilute, particulate-laden, density-driven flows - turbidity currents - are a predominant mechanism for transporting sediment from source to sink in deep marine environments. These flows sculpt channels on the seafloor and, as evidenced by a wealth of bathymetric data, can travel for >1000km, forming some of the largest sedimentary landforms on the planet. For turbidity currents to travel such large dsitances, sediment must be self-maintained in suspension, i.e., be in a state of autosuspension. It has been shown that such self-maintained sediment suspensions can only occur whilst inertial forces are greater than gravitational forces, entailing supercritical flow. This conclusion is paradoxical, as inertia dominated flows rapidly entrain fluid, thereby thickening and slowing to become subcritical. However, current theory can only truly be applied to the proximal upper slope regions of seafloor channels where incised flows are fully confined. This contrasts with the distal reaches of long run out turbidity current systems, where the flow is only partially confined through self-channelization. Here it is shown that overspill of partially confined flow has a significant effect on the hydro- and morphodynamics of turbidity current systems. A new model is derived that shows that channel overspill acts to negate the effects of ambient fluid entrainment: a dynamic balance that limits increases in flow depth and maintains supercritical flow throughout the channel. In the new model mass, momentum and energy conservation is modulated by flow overspill onto channel banks, necessarily requiring description of the vertical structure of the flow. Analysis of continuously stratified steady state flow dynamics shows that the integration of overspill and stratification is necessary to enable maintained autosuspension and thus predict the ultra-long run-out of turbidity currents.

Azimuthal swirl in liquid metal electrodes and batteries

NASA Astrophysics Data System (ADS)

Ashour, Rakan; Kelley, Douglas

2016-11-01

Liquid metal batteries consist of two molten metals with different electronegativity separated by molten salt. In these batteries, critical performance related factors such as the limiting current density are governed by fluid mixing in the positive electrode. In this work we present experimental results of a swirling flow in a layer of molten lead-bismuth alloy driven by electrical current. Using in-situ ultrasound velocimetery, we show that poloidal circulation appears at low current density, whereas azimuthal swirl becomes dominant at higher current density. The presence of thermal gradients produces buoyant forces, which are found to compete with those produced by current injection. Taking the ratio of the characteristic electromagnetic to buoyant flow velocity, we are able to predict the current density at which the flow becomes electromagnetically driven. Scaling arguments are also used to show that swirl is generated through self-interaction between the electrical current in the electrode with its own magnetic field.

Diffusion and transport in locally disordered driven lattices

NASA Astrophysics Data System (ADS)

Wulf, Thomas; Okupnik, Alexander; Schmelcher, Peter

2016-09-01

We study the effect of disorder on the particle density evolution in a classical Hamiltonian driven lattice setup. If the disorder is localized within a finite sub-domain of the lattice, the emergence of strong tails in the density distribution which even increases towards larger positions is shown, thus yielding a highly non-Gaussian particle density evolution. As the key underlying mechanism, we identify the conversion between different components of the unperturbed systems mixed phase space which is induced by the disorder. Based on the introduction of individual conversion rates between chaotic and regular components, a theoretical model is developed which correctly predicts the scaling of the particle density. The effect of disorder on the transport properties is studied where a significant enhancement of the transport for cases of localized disorder is shown, thereby contrasting strongly the merely weak modification of the transport for global disorder.

Spatial Analysis of Geothermal Resource Potential in New York and Pennsylvania: A Stratified Kriging Approach

NASA Astrophysics Data System (ADS)

Smith, J. D.; Whealton, C. A.; Stedinger, J. R.

2014-12-01

Resource assessments for low-grade geothermal applications employ available well temperature measurements to determine if the resource potential is sufficient for supporting district heating opportunities. This study used a compilation of bottomhole temperature (BHT) data from recent unconventional shale oil and gas wells, along with legacy oil, gas, and storage wells, in Pennsylvania (PA) and New York (NY). Our study's goal was to predict the geothermal resource potential and associated uncertainty for the NY-PA region using kriging interpolation. The dataset was scanned for outliers, and some observations were removed. Because these wells were drilled for reasons other than geothermal resource assessment, their spatial density varied widely. An exploratory spatial statistical analysis revealed differences in the spatial structure of the geothermal gradient data (the kriging semi-variogram and its nugget variance, shape, sill, and the degree of anisotropy). As a result, a stratified kriging procedure was adopted to better capture the statistical structure of the data, to generate an interpolated surface, and to quantify the uncertainty of the computed surface. The area was stratified reflecting different physiographic provinces in NY and PA that have geologic properties likely related to variations in the value of the geothermal gradient. The kriging prediction and the variance-of-prediction were determined for each province by the generation of a semi-variogram using only the wells that were located within that province. A leave-one-out cross validation (LOOCV) was conducted as a diagnostic tool. The results of stratified kriging were compared to kriging using the whole region to determine the impact of stratification. The two approaches provided similar predictions of the geothermal gradient. However, the variance-of-prediction was different. The stratified approach is recommended because it gave a more appropriate site-specific characterization of uncertainty based upon a more realistic description of the statistical structure of the data given the geologic characteristics of each province.

Electrophoretic interactions and aggregation of colloidal biological particles

NASA Technical Reports Server (NTRS)

Davis, Robert H.; Nichols, Scott C.; Loewenberg, Michael; Todd, Paul

1994-01-01

The separation of cells or particles from solution has traditionally been accomplished with centrifuges or by sedimentation; however, many particles have specific densities close to unity, making buoyancy-driven motion slow or negligible, but most cells and particles carry surface charges, making them ideal for electrophoretic separation. Both buoyancy-driven and electrophoretic separation may be influenced by hydrodynamic interactions and aggregation of neighboring particles. Aggregation by electrophoresis was analyzed for two non-Brownian particles with different zeta potentials and thin double layers migrating through a viscous fluid. The results indicate that the initial rate of electrophoretically-driven aggregation may exceed that of buoyancy-driven aggregation, even under conditions in which buoyancy-driven relative motion of noninteracting particles is dominant.

Evaluation of a Stratified National Breast Screening Program in the United Kingdom: An Early Model-Based Cost-Effectiveness Analysis.

PubMed

Gray, Ewan; Donten, Anna; Karssemeijer, Nico; van Gils, Carla; Evans, D Gareth; Astley, Sue; Payne, Katherine

2017-09-01

To identify the incremental costs and consequences of stratified national breast screening programs (stratified NBSPs) and drivers of relative cost-effectiveness. A decision-analytic model (discrete event simulation) was conceptualized to represent four stratified NBSPs (risk 1, risk 2, masking [supplemental screening for women with higher breast density], and masking and risk 1) compared with the current UK NBSP and no screening. The model assumed a lifetime horizon, the health service perspective to identify costs (£, 2015), and measured consequences in quality-adjusted life-years (QALYs). Multiple data sources were used: systematic reviews of effectiveness and utility, published studies reporting costs, and cohort studies embedded in existing NBSPs. Model parameter uncertainty was assessed using probabilistic sensitivity analysis and one-way sensitivity analysis. The base-case analysis, supported by probabilistic sensitivity analysis, suggested that the risk stratified NBSPs (risk 1 and risk-2) were relatively cost-effective when compared with the current UK NBSP, with incremental cost-effectiveness ratios of £16,689 per QALY and £23,924 per QALY, respectively. Stratified NBSP including masking approaches (supplemental screening for women with higher breast density) was not a cost-effective alternative, with incremental cost-effectiveness ratios of £212,947 per QALY (masking) and £75,254 per QALY (risk 1 and masking). When compared with no screening, all stratified NBSPs could be considered cost-effective. Key drivers of cost-effectiveness were discount rate, natural history model parameters, mammographic sensitivity, and biopsy rates for recalled cases. A key assumption was that the risk model used in the stratification process was perfectly calibrated to the population. This early model-based cost-effectiveness analysis provides indicative evidence for decision makers to understand the key drivers of costs and QALYs for exemplar stratified NBSP. Copyright © 2017 International Society for Pharmacoeconomics and Outcomes Research (ISPOR). Published by Elsevier Inc. All rights reserved.

Nonequilibrium Fluctuations and Enhanced Diffusion of a Driven Particle in a Dense Environment

NASA Astrophysics Data System (ADS)

Illien, Pierre; Bénichou, Olivier; Oshanin, Gleb; Sarracino, Alessandro; Voituriez, Raphaël

2018-05-01

We study the diffusion of a tracer particle driven out of equilibrium by an external force and traveling in a dense environment of arbitrary density. The system evolves on a discrete lattice and its stochastic dynamics is described by a master equation. Relying on a decoupling approximation that goes beyond the naive mean-field treatment of the problem, we calculate the fluctuations of the position of the tracer around its mean value on a lattice of arbitrary dimension, and with different boundary conditions. We reveal intrinsically nonequilibrium effects, such as enhanced diffusivity of the tracer induced by both the crowding interactions and the external driving. We finally consider the high-density and low-density limits of the model and show that our approximation scheme becomes exact in these limits.

Coupled Leidenfrost states as a monodisperse granular clock

NASA Astrophysics Data System (ADS)

Liu, Rui; Yang, Mingcheng; Chen, Ke; Hou, Meiying; To, Kiwing

2016-08-01

Using an event-driven molecular dynamics simulation, we show that simple monodisperse granular beads confined in coupled columns may oscillate as a different type of granular clock. To trigger this oscillation, the system needs to be driven against gravity into a density-inverted state, with a high-density clustering phase supported from below by a gaslike low-density phase (Leidenfrost effect) in each column. Our analysis reveals that the density-inverted structure and the relaxation dynamics between the phases can amplify any small asymmetry between the columns, and lead to a giant oscillation. The oscillation occurs only for an intermediate range of the coupling strength, and the corresponding phase diagram can be universally described with a characteristic height of the density-inverted structure. A minimal two-phase model is proposed and a linear stability analysis shows that the triggering mechanism of the oscillation can be explained as a switchable two-parameter Andronov-Hopf bifurcation. Numerical solutions of the model also reproduce similar oscillatory dynamics to the simulation results.

Effects of slope on the dynamics of dilute pyroclastic density currents from May 18th, 1980 Mt. St. Helens eruption

NASA Astrophysics Data System (ADS)

Bendana, S.; Self, S.; Dufek, J.

2012-12-01

The infamous, May 18th, 1980 eruption of Mt St Helens in the state of Washington produced several episodes of pyroclastic density currents (PDCs) including the initial lateral blast, which traveled nearly 30 km, and later PDCs, which filled in the area up to 8 km north of the volcano. The focus of this research is on the later PDCs, which differed from the lateral blast in that they have a higher particle concentration and filled in the topography up to 40 m. While the concentrated portions of the afternoon PDCs followed deep topographic drainages down the steep flanks of the volcano, the dilute overriding cloud partially decoupled to develop fully dilute, turbulent PDCs on the flanks of the volcano (Beeson, D.L. 1988. Proximal Flank Facies of the May 18, 1980 Ignimbrite: Mt. St. Helens, Washington.). The dilute PDCs deposited thin, cross-stratified and stratified pyroclastic deposits, known as the proximal bedded deposits, which differ greatly in depositional characteristics from the thick, massive, poorly-sorted, block-rich deposits associated with the more concentrated portions of the flow. We explore the influence of topography on the formation of these dilute currents and influence of slope on the currents transport and depositional mechanisms. The deposits on steeper slopes (>15°) are fines depleted relative to the proximal bedded deposits on shallower slopes (<15°). Bedform amplitude and wavelength increase with increasing slope, as does the occurrence of regressive dunes. Increasing slope causes an increase in flow velocity and thus an increase in flow turbulence. The fines depleted deposits suggest that fine ash elutriation is more efficient in flows with stronger turbulence. The longer wavelength and amplitudes suggest that bedform morphology is directly related to flow velocity, an important finding since the controls on bedform wavelength and amplitude in density stratified flows remains poorly constrained. The occurrence of regressive dunes, often interpreted as high flow-regime bedforms, on steeper slopes relative to progressive dunes on shallower slopes further attests to the control of velocity and flow regime on bedform morphology. Samples collected from recently exposed deposits and analyzed by grain size measurements, density analyses, and crystal morphoscopy studies further assess modes of origin and transport of dilute PDCs. The collected data will be used to validate numerical models that attempt to quantify the hazards of decoupled, dilute PDCs.

Distribution and exploitation of Nile perch Lates niloticus in relation to stratification in Lake Victoria, East Africa

USGS Publications Warehouse

Taabu-Munyaho, A.; Kayanda, Robert J.; Everson, Inigo; Grabowski, Timothy B.; Marteinsdóttir, Gudrún

2013-01-01

Stratification restricts habitable areas forcing fish to balance between favourable temperature and minimum dissolved oxygen requirements. Acoustic surveys conducted during the stratified and isothermal periods on tropical Lake Victoria indicated that stratification of temperature and dissolved oxygen (DO) affected vertical distribution of Nile perch. There was higher mean temperature (25.6 ± 0.5 °C) and lower DO (6.4 ± 1.8 mg/l) during stratified period compared to the isothermal period (mean temperature 24.9 ± 0.3 °C; mean DO 7.3 ± 0.6 mg/l). Higher mean densities of Nile perch were recorded in the coastal (0.44 ± 0.03) and deep (0.27 ± 0.02 g/m3) strata during the stratified compared to the isothermal season (coastal: 0.24 ± 0.01; deep: 0.12 ± 0.02 g/m3). In addition, Nile perch density in the upper 0–40 m depth layers in the coastal and deep strata increased by over 50% from the isothermal to the stratified season. Daily landings from 65 motorised fishing boats between October 2008 and September 2010 show higher mean catch (26.29 ± 0.17 kg/boat/day) during stratified compared to the isothermal (23.59 ± 0.15) season. Thermal stratification apparently compresses the habitat available to Nile perch and can potentially result in higher exploitation. Managers should evaluate the potential benefits of instituting closed seasons during the stratified period, and stock assessment models should take into account the seasonal niche compression.

On the lifetime of a pancake anticyclone in a rotating stratified flow

NASA Astrophysics Data System (ADS)

Facchini, Giulio; Le Bars, Michael

2016-11-01

We present an experimental study of the time evolution of an isolated anticyclonic pancake vortex in a laboratory rotating stratified flow. Motivations come from the variety of compact anticyclones observed to form and persist for a strikingly long lifetime in geophysical and astrophysical settings combining rotation and stratification. We generate anticyclones by injecting a small amount of isodense fluid at the center of a rotating tank filled with salty water linearly stratified in density. Our two control parameters are the Coriolis parameter f and the Brunt-Väisälä frequency N. We observe that anticyclones always slowly decay by viscous diffusion, spreading mainly in the horizontal direction irrespective of the initial aspect ratio. This behavior is correctly explained by a linear analytical model in the limit of small Rossby and Ekman numbers, where density and velocity equations reduce to a single equation for the pressure. Direct numerical simulations further confirm the theoretical predictions. Notably, they show that the azimuthal shear stress generates secondary circulations, which advect the density anomaly: this mechanism is responsible for the slow time evolution, rather than the classical viscous dissipation of the azimuthal kinetic energy.

Numerical Simulations of a Multiscale Model of Stratified Langmuir Circulation

NASA Astrophysics Data System (ADS)

Malecha, Ziemowit; Chini, Gregory; Julien, Keith

2012-11-01

Langmuir circulation (LC), a prominent form of wind and surface-wave driven shear turbulence in the ocean surface boundary layer (BL), is commonly modeled using the Craik-Leibovich (CL) equations, a phase-averaged variant of the Navier-Stokes (NS) equations. Although surface-wave filtering renders the CL equations more amenable to simulation than are the instantaneous NS equations, simulations in wide domains, hundreds of times the BL depth, currently earn the ``grand challenge'' designation. To facilitate simulations of LC in such spatially-extended domains, we have derived multiscale CL equations by exploiting the scale separation between submesoscale and BL flows in the upper ocean. The numerical algorithm for simulating this multiscale model resembles super-parameterization schemes used in meteorology, but retains a firm mathematical basis. We have validated our algorithm and here use it to perform multiscale simulations of the interaction between LC and upper ocean density stratification. ZMM, GPC, KJ gratefully acknowledge funding from NSF CMG Award 0934827.

GRAVITATIONAL CONTRACTION VERSUS SUPERNOVA DRIVING AND THE ORIGIN OF THE VELOCITY DISPERSION–SIZE RELATION IN MOLECULAR CLOUDS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ibáñez-Mejía, Juan C.; Mac Low, Mordecai-Mark; Klessen, Ralf S.

Molecular cloud (MC) observations show that clouds have non-thermal velocity dispersions that scale with the cloud size as σ ∝ R {sup 1/2} at a constant surface density, and for varying surface density scale with both the cloud’s size and surface density, σ {sup 2} ∝ R Σ. The energy source driving these chaotic motions remains poorly understood. We describe the velocity dispersions observed in a cloud population formed in a numerical simulation of a magnetized, stratified, supernova (SN)-driven, interstellar medium, including diffuse heating and radiative cooling, before and after we include the effects of the self-gravity of the gas.more » We compare the relationships between velocity dispersion, size, and surface density measured in the simulated cloud population to those found in observations of Galactic MCs. Our simulations prior to the onset of self-gravity suggest that external SN explosions alone do not drive turbulent motions of the observed magnitudes within dense clouds. On the other hand, self-gravity induces non-thermal motions as gravitationally bound clouds begin to collapse in our model, approaching the observed relations between velocity dispersion, size, and surface density. Energy conservation suggests that the observed behavior is consistent with the kinetic energy being proportional to the gravitational energy. However, the clouds in our model show no sign of reaching a stable equilibrium state at any time, even for strongly magnetized clouds. We conclude that gravitationally bound MCs are always in a state of gravitational contraction and their properties are a natural result of this chaotic collapse. In order to agree with observed star formation efficiencies, this process must be terminated by the early destruction of the clouds, presumably from internal stellar feedback.« less

Theory and simulation of buoyancy-driven convection around growing protein crystals in microgravity.

PubMed

Carotenuto, L; Cartwright, J H E; Castagnolo, D; Garcia Ruiz, J M; Otalora, F

2002-01-01

We present an order-of-magnitude analysis of the Navier-Stokes equations in a time-dependent, incompressible and Boussinesq formulation. The hypothesis employed of two different length scales allows one to determine the different flow regimes on the basis of the geometrical and thermodynamical parameters alone, without solving the Navier-Stokes equations. The order-of-magnitude analysis is then applied to the field of protein crystallization, and to the flow field around a crystal, where the driving forces are solutal buoyancy-driven convection, from density dependence on species concentration, and sedimentation caused by the different densities of the crystal and the protein solution. The main result of this paper is to provide predictions of the conditions in which a crystal is growing in a convective regime, rather than in the ideal diffusive state, even under the typical microgravity conditions of space platforms.

The Near Wake of Bluff Bodies in Stratified Fluids and the Emergence of Late Wake Characteristics

DTIC Science & Technology

2010-10-29

including suggestions for reducing this burden to Department of Defense. Washington Headquarters Services, Directorate for Information Operations and...represents the orthnormal coordinate vectors in a Cartesian coordinate system , u = i^ei is the velocity vector field, P is pressure, p is the density, and... different characteristics depending upon the Reynolds number, the Froude number, and possibly the diffusivity [22] of the flow. In turn, the

A Model for the Thermal and Chemical Evolution of the Moon's Interior: Implications for the Onset of Mare Volcanism

NASA Technical Reports Server (NTRS)

Hess, Paul C.; Parmentier, E. M.

1995-01-01

Crystallization of the lunar magma ocean creates a chemically stratified Moon consisting of an anorthositic crust and magma ocean cumulates overlying the primitive lunar interior. Within the magma ocean cumulates the last liquids to crystallize form dense, ilmenite-rich cumulates that contain high concentrations of incompatible radioactive elements. The underlying olivine-orthopyroxene cumulates are also stratified with later crystallized, denser, more Fe-rich compositions at the top. This paper explores the chemical and thermal consequences of an internal evolution model accounting for the possible role of these sources of chemical buoyancy. Rayleigh-Taylor instability causes the dense ilmenite-rich cumulate layer and underlying Fe-rich cumulates to sink toward the center of the Moon, forming a dense lunar core. After this overturn, radioactive heating within the ilmenite-rich cumulate core heats the overlying mantle, causing it to melt. In this model, the source region for high-TiO2 mare basalts is a convectively mixed layer above the core-mantle boundary which would contain small and variable amounts of admixed ilmenite and KREEP. This deep high-pressure melting, as required for mare basalts, occurs after a reasonable time interval to explain the onset of mare basalt volcanism if the content of radioactive elements in the core and the chemical density gradients above the core are sufficiently high but within a range of values that might have been present in the Moon. Regardless of details implied by particular model parameters, gravitational overturn driven by the high density of magma ocean Fe-rich cumulates should concentrate high-TiO2 mare basalt sources, and probably a significant fraction of radioactive heating, toward the center of the Moon. This will have important implications for both the thermal evolution of the Moon and for mare basalt genesis.

Design and simulation of stratified probability digital receiver with application to the multipath communication

NASA Technical Reports Server (NTRS)

Deal, J. H.

1975-01-01

One approach to the problem of simplifying complex nonlinear filtering algorithms is through using stratified probability approximations where the continuous probability density functions of certain random variables are represented by discrete mass approximations. This technique is developed in this paper and used to simplify the filtering algorithms developed for the optimum receiver for signals corrupted by both additive and multiplicative noise.

Convection Enhances Mixing in the Southern Ocean

NASA Astrophysics Data System (ADS)

Sohail, Taimoor; Gayen, Bishakhdatta; Hogg, Andrew McC.

2018-05-01

Mixing efficiency is a measure of the energy lost to mixing compared to that lost to viscous dissipation. In a turbulent stratified fluid the mixing efficiency is often assumed constant at η = 0.2, whereas with convection it takes values closer to 1. The value of mixing efficiency when both stratified shear flow and buoyancy-driven convection are active remains uncertain. We use a series of numerical simulations to determine the mixing efficiency in an idealized Southern Ocean model. The model is energetically closed and fully resolves convection and turbulence such that mixing efficiency can be diagnosed. Mixing efficiency decreases with increasing wind stress but is enhanced by turbulent convection and by large thermal gradients in regions with a strongly stratified thermocline. Using scaling theory and the model results, we predict an overall mixing efficiency for the Southern Ocean that is significantly greater than 0.2 while emphasizing that mixing efficiency is not constant.

Observations and analysis of a stratification-destratification event in a tropical estuary

NASA Astrophysics Data System (ADS)

Uncles, R. J.; Ong, J. E.; Gong, W. K.

1990-11-01

A data set comprising 31 continuous tidal cycles was collected in the Sungai Merbok Estuary, Malaysia, in June 1987 as part of an ecological study of nutrient fluxes from a tropical mangrove estuary. Currents, salinity and salinity stratification at a deep-channel (15 m) station near the mouth of the Merbok Estuary showed a pronounced spring-neap variability. The slow currents and weak vertical mixing at neap tides favoured the formation of a stratified water column and generated a neap-spring cycle of water column stabilization and destabilization. A strong stratification event occurred during the period of observations. This was partly driven by a modest freshwater spate which coincided with neap tides. An eddy viscosity-diffusivity model of the stratification, which assumed a constant, longitudinal salinity gradient, demonstrated a pronounced stratification-destratification cycle due to neap-spring variations in vertical mixing. Larger and more realistic stratification was modelled when the estimated, time-varying longitudinal salinity gradient was incorporated. This gradient maximized in response to the peak in freshwater runoff. The measured and modelled density-driven circulations showed qualitative similarities and were of the order of 10 cm s -1 at neap tides. The circulation was weaker during spring tides. The tidally-filtered salt transport due to vertical shear was directed up-estuary and was an order of magnitude smaller during spring tides. The results are discussed in terms of their relevance to mangrove system oceanography.

SUPERGRANULATION AS THE LARGEST BUOYANTLY DRIVEN CONVECTIVE SCALE OF THE SUN

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cossette, Jean-Francois; Rast, Mark P.

The origin of solar supergranulation remains a mystery. Unlike granulation, the size of which is comparable to both the thickness of the radiative boundary layer and local scale-height in the photosphere, supergranulation does not reflect any obvious length scale of the solar convection zone. Moreover, recent observations of flows in the photosphere using Doppler imaging or correlation or feature tracking show a monotonic decrease in horizontal flow power at scales larger than supergranulation. Both local area and global spherical shell simulations of solar convection by contrast show the opposite, an increase in horizontal flow amplitudes to a low wavenumber. Wemore » examine these disparities and investigate how the solar supergranulation may arise as a consequence of nonlocal heat transport by cool diving plumes. Using three-dimensional anelastic simulations with surface driving, we show that the kinetic energy of the largest convective scales in the upper layers of a stratified domain reflects the depth of transition from strong buoyant driving to adiabatic stratification below caused by the dilution of the granular downflows. This depth is quite shallow because of the rapid increase of the mean density below the photosphere. We interpret the observed monotonic decrease in solar convective power at scales larger than supergranulation to be a consequence of this rapid transition, with the supergranular scale the largest buoyantly driven mode of convection in the Sun.« less

Numerical investigation of split flows by gravity currents into two-layered stratified water bodies

NASA Astrophysics Data System (ADS)

Cortés, A.; Wells, M. G.; Fringer, O. B.; Arthur, R. S.; Rueda, F. J.

2015-07-01

The behavior of a two-dimensional (2-D) gravity current impinging upon a density step in a two-layered stratified basin is analyzed using a high-resolution Reynolds-Averaged Navier-Stokes model. The gravity current splits at the density step, and the portion of the buoyancy flux becoming an interflow is largely controlled by the vertical distribution of velocity and density within the gravity current and the magnitude of the density step between the two ambient layers. This is in agreement with recent laboratory observations. The strongest changes in the ambient density profiles occur as a result of the impingement of supercritical currents with strong density contrasts, for which a large portion of the gravity current detaches from the bottom and becomes an interflow. We characterize the current partition process in the simulated experiments using the densimetric Froude number of the current (Fr) across the density step (upstream and downstream). When underflows are formed, more supercritical currents are observed downstream of the density step compared to upstream (Fru < Frd), and thus, stronger mixing of the current with the ambient water downstream. However, when split flows and interflows are formed, smaller Fr values are identified after the current crosses the density step (Fru > Frd), which indicates lower mixing between the current and ambient water after the impingement due to the significant stripping of interfacial material at the density step.

Maximum current density and beam brightness achievable by laser-driven electron sources

NASA Astrophysics Data System (ADS)

Filippetto, D.; Musumeci, P.; Zolotorev, M.; Stupakov, G.

2014-02-01

This paper discusses the extension to different electron beam aspect ratio of the Child-Langmuir law for the maximum achievable current density in electron guns. Using a simple model, we derive quantitative formulas in good agreement with simulation codes. The new scaling laws for the peak current density of temporally long and transversely narrow initial beam distributions can be used to estimate the maximum beam brightness and suggest new paths for injector optimization.

Building a Better Model: A Personalized Breast Cancer Risk Model Incorporating Breast Density to Stratify Risk and Improve Application of Resources

DTIC Science & Technology

2013-10-01

a GE unit and 100 images from a Hologic unit. These were reviewed during Dr. Harvey’s visit to Toronto October 2012. The ...patient underwent the standard of practice 4-view mammogram. Following this, a different technologist obtained a second craniocaudal image of the left...project and one related to a current event. Representatives from the project were present to provide information at the Charlottesville Four

Turbulence Statistics of a Buoyant Jet in a Stratified Environment

NASA Astrophysics Data System (ADS)

McCleney, Amy Brooke

Using non-intrusive optical diagnostics, turbulence statistics for a round, incompressible, buoyant, and vertical jet discharging freely into a stably linear stratified environment is studied and compared to a reference case of a neutrally buoyant jet in a uniform environment. This is part of a validation campaign for computational fluid dynamics (CFD). Buoyancy forces are known to significantly affect the jet evolution in a stratified environment. Despite their ubiquity in numerous natural and man-made flows, available data in these jets are limited, which constrain our understanding of the underlying physical processes. In particular, there is a dearth of velocity field data, which makes it challenging to validate numerical codes, currently used for modeling these important flows. Herein, jet near- and far-field behaviors are obtained with a combination of planar laser induced fluorescence (PLIF) and multi-scale time-resolved particle image velocimetry (TR-PIV) for Reynolds number up to 20,000. Deploying non-intrusive optical diagnostics in a variable density environment is challenging in liquids. The refractive index is strongly affected by the density, which introduces optical aberrations and occlusions that prevent the resolution of the flow. One solution consists of using index matched fluids with different densities. Here a pair of water solutions - isopropanol and NaCl - are identified that satisfy these requirements. In fact, they provide a density difference up to 5%, which is the largest reported for such fluid pairs. Additionally, by design, the kinematic viscosities of the solutions are identical. This greatly simplifies the analysis and subsequent simulations of the data. The spectral and temperature dependence of the solutions are fully characterized. In the near-field, shear layer roll-up is analyzed and characterized as a function of initial velocity profile. In the far-field, turbulence statistics are reported for two different scales, one capturing the entire jet at near Taylor microscale resolution, and the other, thanks to the careful refractive index matching of the liquids, resolving the Taylor scale at near Kolmogorov scale resolution. This is accomplished using a combination of TR-PIV and long-distance micro-PIV. The turbulence statistics obtained at various downstream locations and magnifications are obtained for density differences of 0%, 1%, and 3%. To validate the experimental methodology and provide a reference case for validation, the effect of initial velocity profile on the neutrally buoyant jet in the self-preserving regime is studied at two Reynolds numbers of 10,000 and 20,000. For the neutrally buoyant jet, it is found that independent of initial conditions the jet follows a self-similar behavior in the far-field; however, the spreading rate is strongly dependent on initial velocity profile. High magnification analysis at the small turbulent length scales shows a flow field where the mean statistics compare well to the larger field of view case. Investigation of the near-field shows the jet is strongly influenced by buoyancy, where an increase in vortex ring formation frequency and number of pairings occur. The buoyant jet with a 1% density difference shows an alteration of the centerline velocity decay, but the radial distribution of the mean axial velocity collapses well at all measurement locations. Jet formation dramatically changes for a buoyant jet with a 3% density difference, where the jet reaches a terminal height and spreads out horizontally at its neutral buoyancy location. Analysis of both the mean axial velocity and strain rates show the jet is no longer self-similar; for example, the mean centerline velocity does not decay uniformly as the jet develops. The centerline strain rates at this density difference also show trends which are strongly influenced by the altered centerline velocity. The overall centerline analysis shows that turbulence suppression occurs as a result of the stratification for both the 1% and 3% density difference. Analysis on the kinetic energy budget shows that the mean convection, production, transportation, and dissipation of energy is altered from stratification. High resolution data of the jet enable flow structures to be captured in the neutrally buoyant region of the flow. Vortices of different sizes are identified. Longer data sets are necessary to perform a statistical analysis of their distribution and to compare them to homogeneous environment case. This multi-scale analysis shows potential for studying energy transfer between length scales.

Impact of convection and resistivity on angular momentum transport in dwarf novae

NASA Astrophysics Data System (ADS)

Scepi, N.; Lesur, G.; Dubus, G.; Flock, M.

2018-01-01

The eruptive cycles of dwarf novae are thought to be due to a thermal-viscous instability in the accretion disk surrounding the white dwarf. This model has long been known to imply enhanced angular momentum transport in the accretion disk during outburst. This is measured by the stress to pressure ratio α, with α ≈ 0.1 required in outburst compared to α ≈ 0.01 in quiescence. Such an enhancement in α has recently been observed in simulations of turbulent transport driven by the magneto-rotational instability (MRI) when convection is present, without requiring a net magnetic flux. We independently recover this result by carrying out PLUTO magnetohydrodynamic (MHD) simulations of vertically stratified, radiative, shearing boxes with the thermodynamics and opacities appropriate to dwarf novae. The results are robust against the choice of vertical boundary conditions. The thermal equilibrium solutions found by the simulations trace the well-known S-curve in the density-temperature plane that constitutes the core of the disk thermal-viscous instability model. We confirm that the high values of α ≈ 0.1 occur near the tip of the hot branch of the S-curve, where convection is active. However, we also present thermally stable simulations at lower temperatures that have standard values of α ≈ 0.03 despite the presence of vigorous convection. We find no simple relationship between α and the strength of the convection, as measured by the ratio of convective to radiative flux. The cold branch is only very weakly ionized so, in the second part of this work, we studied the impact of non-ideal MHD effects on transport. Ohmic dissipation is the dominant effect in the conditions of quiescent dwarf novae. We include resistivity in the simulations and find that the MRI-driven transport is quenched (α ≈ 0) below the critical density at which the magnetic Reynolds number Rm ≤ 104. This is problematic because the X-ray emission observed in quiescent systems requires ongoing accretion onto the white dwarf. We verify that these X-rays cannot self-sustain MRI-driven turbulence by photo-ionizing the disk and discuss possible solutions to the issue of accretion in quiescence.

Wavefront-sensor-based electron density measurements for laser-plasma accelerators.

PubMed

Plateau, G R; Matlis, N H; Geddes, C G R; Gonsalves, A J; Shiraishi, S; Lin, C; van Mourik, R A; Leemans, W P

2010-03-01

Characterization of the electron density in laser produced plasmas is presented using direct wavefront analysis of a probe laser beam. The performance of a laser-driven plasma-wakefield accelerator depends on the plasma wavelength and hence on the electron density. Density measurements using a conventional folded-wave interferometer and using a commercial wavefront sensor are compared for different regimes of the laser-plasma accelerator. It is shown that direct wavefront measurements agree with interferometric measurements and, because of the robustness of the compact commercial device, offer greater phase sensitivity and straightforward analysis, improving shot-to-shot plasma density diagnostics.

Wavefront-sensor-based electron density measurements for laser-plasma accelerators

DOE Office of Scientific and Technical Information (OSTI.GOV)

Plateau, Guillaume; Matlis, Nicholas; Geddes, Cameron

2010-02-20

Characterization of the electron density in laser produced plasmas is presented using direct wavefront analysis of a probe laser beam. The performance of a laser-driven plasma-wakefield accelerator depends on the plasma wavelength, hence on the electron density. Density measurements using a conventional folded-wave interferometer and using a commercial wavefront sensor are compared for different regimes of the laser-plasma accelerator. It is shown that direct wavefront measurements agree with interferometric measurements and, because of the robustness of the compact commercial device, have greater phase sensitivity, straightforward analysis, improving shot-to-shot plasma-density diagnostics.

The evolutionary and behavioral modification of consumer responses to environmental change.

PubMed

Abrams, Peter A

2014-02-21

How will evolution or other forms of adaptive change alter the response of a consumer species' population density to environmentally driven changes in population growth parameters? This question is addressed by analyzing some simple consumer-resource models to separate the ecological and evolutionary components of the population's response. Ecological responses are always decreased population size, but evolution of traits that have effects on both resource uptake rate and another fitness-related parameter may magnify, offset, or reverse this population decrease. Evolution can change ecologically driven decreases in population size to increases; this is likely when: (1) resources are initially below the density that maximizes resource growth, and (2) the evolutionary response decreases the consumer's resource uptake rate. Evolutionary magnification of the ecological decreases in population size can occur when the environmental change is higher trait-independent mortality. Such evolution-driven decreases are most likely when uptake-rate traits increase and the resource is initially below its maximum growth density. It is common for the difference between the new eco-evolutionary equilibrium and the new ecological equilibrium to be larger than that between the original and new ecological equilibrium densities. The relative magnitudes of ecological and evolutionary effects often depend sensitively on the magnitude of the environmental change and the nature of resource growth. © 2013 Elsevier Ltd. All rights reserved.

Stratified turbulent Bunsen flames: flame surface analysis and flame surface density modelling

NASA Astrophysics Data System (ADS)

Ramaekers, W. J. S.; van Oijen, J. A.; de Goey, L. P. H.

2012-12-01

In this paper it is investigated whether the Flame Surface Density (FSD) model, developed for turbulent premixed combustion, is also applicable to stratified flames. Direct Numerical Simulations (DNS) of turbulent stratified Bunsen flames have been carried out, using the Flamelet Generated Manifold (FGM) reduction method for reaction kinetics. Before examining the suitability of the FSD model, flame surfaces are characterized in terms of thickness, curvature and stratification. All flames are in the Thin Reaction Zones regime, and the maximum equivalence ratio range covers 0.1⩽φ⩽1.3. For all flames, local flame thicknesses correspond very well to those observed in stretchless, steady premixed flamelets. Extracted curvature radii and mixing length scales are significantly larger than the flame thickness, implying that the stratified flames all burn in a premixed mode. The remaining challenge is accounting for the large variation in (subfilter) mass burning rate. In this contribution, the FSD model is proven to be applicable for Large Eddy Simulations (LES) of stratified flames for the equivalence ratio range 0.1⩽φ⩽1.3. Subfilter mass burning rate variations are taken into account by a subfilter Probability Density Function (PDF) for the mixture fraction, on which the mass burning rate directly depends. A priori analysis point out that for small stratifications (0.4⩽φ⩽1.0), the replacement of the subfilter PDF (obtained from DNS data) by the corresponding Dirac function is appropriate. Integration of the Dirac function with the mass burning rate m=m(φ), can then adequately model the filtered mass burning rate obtained from filtered DNS data. For a larger stratification (0.1⩽φ⩽1.3), and filter widths up to ten flame thicknesses, a β-function for the subfilter PDF yields substantially better predictions than a Dirac function. Finally, inclusion of a simple algebraic model for the FSD resulted only in small additional deviations from DNS data, thereby rendering this approach promising for application in LES.

Numerical Simulations of Upstream Propagating Solitary Waves and Wave Breaking In A Stratified Fjord

NASA Astrophysics Data System (ADS)

Stastna, M.; Peltier, W. R.

In this talk we will discuss ongoing numerical modeling of the flow of a stratified fluid over large scale topography motivated by observations in Knight Inlet, a fjord in British Columbia, Canada. After briefly surveying the work done on the topic in the past we will discuss our latest set of simulations in which we have observed the gener- ation and breaking of three different types of nonlinear internal waves in the lee of the sill topography. The first type of wave observed is a large lee wave in the weakly strat- ified main portion of the water column, The second is an upward propagating internal wave forced by topography that breaks in the strong, near-surface pycnocline. The third is a train of upstream propagating solitary waves that, in certain circumstances, form as breaking waves consisting of a nearly solitary wave envelope and a highly unsteady core near the surface. Time premitting, we will comment on the implications of these results for our long term goal of quantifying tidally driven mixing in Knight Inlet.

Reduction of MRI-targeted biopsies in men with low-risk prostate cancer on active surveillance by stratifying to PI-RADS and PSA-density, with different thresholds for significant disease.

PubMed

Schoots, Ivo G; Osses, Daniel F; Drost, Frank-Jan H; Verbeek, Jan F M; Remmers, Sebastiaan; van Leenders, Geert J L H; Bangma, Chris H; Roobol, Monique J

2018-02-01

The fear of undergrading prostate cancer (PCa) in men on active surveillance (AS) have led to strict criteria for monitoring, which have resulted in good long-term cancer-specific survival, proving the safety of this approach. Reducing undergrading, MRI-targeted biopsies are increasingly used in men with low-risk disease despite their undefined role yet. The objective of this study is to investigate the rate of upgrading using MRI-targeted biopsies in men with low-risk disease on AS, stratified on the basis of PI-RADS and PSA-density, with the aim to reduce potential unnecessary repeat biopsy procedures. A total of 331 men were prospectively enrolled following the MRI-PRIAS protocol. MR imaging was according to Prostate Imaging Reporting and Data System (PI-RADSv2) guidelines. Suspicious MRI lesions (PI-RADS 3-5) were additionally targeted by MRI-TRUS fusion biopsies. Outcome measure was upgrading to Gleason score (GS) ≥3+4 with MRI-targeted biopsies, stratified for PI-RADS and PSA-density. In total, 25% (82/331) of men on AS showed upgrading from GS 3+3. Only 3% (11/331) was upgraded to GS ≥8. In 60% (198/331) a suspicious MRI lesion was identified, but in only 41% (82/198) of men upgrading was confirmed. PI-RADS 3, 4 and 5 categorized index lesions, showed upgrading in 30%, 34% and 66% of men, respectively. Stratification to PI-RADS 4-5, instead of PI-RADS 3-5, would have missed a small number of high volume Gleason 4 PCa in PI-RADS 3 category. However, further stratification into PI-RADS 3 lesions and PSA-density <0.15 ng/mL 2 could result in a safe targeted biopsy reduction of 36% in this category, without missing any upgrades. Stratification with the combination of PI-RADS and PSA-density may reduce unnecessary additional MRI biopsy testing. Overall, the high rate of detected upgrading in men on AS may result in an unintended tightening of continuing in AS. Since patients, included under current AS criteria showed extremely favorable outcome, there might be no need to further restrict continuing on AS with MRI and targeted biopsies.

Poor Dietary Guidelines Compliance among Low-Income Women Eligible for Supplemental Nutrition Assistance Program-Education (SNAP-Ed).

PubMed

Jun, Shinyoung; Thuppal, Sowmyanarayanan V; Maulding, Melissa K; Eicher-Miller, Heather A; Savaiano, Dennis A; Bailey, Regan L

2018-03-08

The Supplemental Nutrition Assistance Program-Education (SNAP-Ed) program aims to improve nutritional intakes of low-income individuals (<185% poverty threshold). The objective of this study was to describe the compliance with Dietary Guidelines for Americans (DGA) recommendations for fruits, vegetables, and whole grains among SNAP-Ed eligible ( n = 3142) and ineligible ( n = 3168) adult women (19-70 years) nationwide and SNAP-Ed participating women in Indiana ( n = 2623), using the NHANES 2007-2012 and Indiana SNAP-Ed survey data, respectively. Sensitivity analysis further stratified women by race/ethnicity and by current SNAP participation (<130% poverty threshold). Nationally, lower-income women were less likely to meet the fruit (21% vs. 25%) and vegetable (11% vs. 19%) guidelines than higher-income women, but did not differ on whole grains, which were ~5% regardless of income. The income differences in fruit and vegetable intakes were driven by non-Hispanic whites. Fewer SNAP-Ed-eligible U.S. women met fruit (21% vs. 55%) and whole grain (4% vs. 18%) but did not differ for vegetable recommendations (11% vs. 9%) when compared to Indiana SNAP-Ed women. This same trend was observed among current SNAP participants. Different racial/ethnic group relationships with DGA compliance were found in Indiana compared to the nation. Nevertheless, most low-income women in the U.S. are at risk of not meeting DGA recommendations for fruits (79%), vegetables (89%), and whole grains (96%); SNAP-Ed participants in Indiana had higher compliance with DGA recommendations. Increased consumption of these three critical food groups would improve nutrient density, likely reduce calorie consumption by replacing high calorie choices, and improve fiber intakes.

Hierarchical spatial capture-recapture models: Modeling population density from stratified populations

USGS Publications Warehouse

Royle, J. Andrew; Converse, Sarah J.

2014-01-01

Capture–recapture studies are often conducted on populations that are stratified by space, time or other factors. In this paper, we develop a Bayesian spatial capture–recapture (SCR) modelling framework for stratified populations – when sampling occurs within multiple distinct spatial and temporal strata.We describe a hierarchical model that integrates distinct models for both the spatial encounter history data from capture–recapture sampling, and also for modelling variation in density among strata. We use an implementation of data augmentation to parameterize the model in terms of a latent categorical stratum or group membership variable, which provides a convenient implementation in popular BUGS software packages.We provide an example application to an experimental study involving small-mammal sampling on multiple trapping grids over multiple years, where the main interest is in modelling a treatment effect on population density among the trapping grids.Many capture–recapture studies involve some aspect of spatial or temporal replication that requires some attention to modelling variation among groups or strata. We propose a hierarchical model that allows explicit modelling of group or strata effects. Because the model is formulated for individual encounter histories and is easily implemented in the BUGS language and other free software, it also provides a general framework for modelling individual effects, such as are present in SCR models.

Rollover in volcanic crater lakes: a possible cause for Lake Nyos type disasters

NASA Astrophysics Data System (ADS)

Rice, A.

2000-04-01

Seemingly stably stratified fluids, that is a heavier layer of fluid underlying an upper layer of lighter fluid, can overturn if there is a heat flux through the system. Such events are termed ;rollover; in the engineering literature (occurring for instance in liquid natural gas tanks). They take place as well in lakes and ponds. In all such cases, the stratification starts off with the heavier, more dense fluid underlying lighter. Convection driven mixing at the stratification interface (due to the heat flux) as well as other processes serves to minimize with time the stabilizing density differences, which may eventually cause the stratification to invert. If gas has been contained under pressure in the lower layer, this gas may explosively vent from the fluid as it suddenly rises to the top such as occurs in liquid natural gas tanks where rollover is a hazard. These processes are quantifiable and are applied here to various scenarios that might refer to volcanic crater lakes whose lower layers are charged with volatiles. Provided herein are some examples of calculated conditions and calculated time of evolution leading to rollover with attendant release of gases from supersaturated ascending fluids. These calculations suggest rollover could occur in volcanic lakes. The August 1986 gas release at Lake Nyos is employed as an example. These estimates were made taking note that water is commonly supersaturated in CO2

The Experimental Study of Rayleigh-Taylor Instability using a Linear Induction Motor Accelerator

NASA Astrophysics Data System (ADS)

Yamashita, Nicholas; Jacobs, Jeffrey

2009-11-01

The experiments to be presented utilize an incompressible system of two stratified miscible liquids of different densities that are accelerated in order to produce the Rayleigh-Taylor instability. Three liquid combinations are used: isopropyl alcohol with water, a calcium nitrate solution or a lithium polytungstate solution, giving Atwood numbers of 0.11, 0.22 and 0.57, respectively. The acceleration required to drive the instability is produced by two high-speed linear induction motors mounted to an 8 m tall drop tower. The motors are mounted in parallel and have an effective acceleration length of 1.7 m and are each capable of producing 15 kN of thrust. The liquid system is contained within a square acrylic tank with inside dimensions 76 x76x184 mm. The tank is mounted to an aluminum plate, which is driven by the motors to create constant accelerations in the range of 1-20 g's, though the potential exists for higher accelerations. Also attached to the plate are a high-speed camera and an LED backlight to provide continuous video of the instability. In addition, an accelerometer is used to provide acceleration measurements during each experiment. Experimental image sequences will be presented which show the development of a random three-dimensional instability from an unforced initial perturbation. Measurements of the mixing zone width will be compared with traditional growth models.

Winds and the distribution of nearshore phytoplankton in a stratified lake.

PubMed

Cyr, Hélène

2017-10-01

The distribution of phytoplankton in lakes is notoriously patchy and dynamic, but wind-driven currents and algal buoyancy/motility are thought to determine where algae accumulate. In this study, nearshore phytoplankton were sampled from different parts of a lake basin twice a day for 4-5 consecutive days, in the spring and in late summer, to test whether short-term changes in phytoplankton biomass and community composition can be predicted from wind-driven currents. On windy days, phytoplankton biomass was higher at downwind than at upwind nearshore sites, and the magnitude of this difference increased linearly with increasing wind speed. However, contrary to the generally assumed downwind phytoplankton aggregations, these differences were mostly due to upwelling activity and the dilution of phytoplankton at upwind nearshore sites. The distribution of individual taxa was also related to wind speed, but only during late stratification (except for cryptophytes), and these relationships were consistent with the buoyancy and motility of each group. On windy days, large diatoms and cyanobacteria concentrated upwind, neutrally buoyant taxa (green algae, small diatoms) were homogeneously distributed, and motile taxa (cryptophytes, chrysophytes, dinoflagellates) concentrated downwind. Predictable differences in the biomass and composition of phytoplankton communities could affect the efficiency of trophic transfers in nearshore areas. Copyright © 2017 Elsevier Ltd. All rights reserved.

Stratification Modelling of Key Bacterial Taxa Driven by Metabolic Dynamics in Meromictic Lakes.

PubMed

Zhu, Kaicheng; Lauro, Federico M; Su, Haibin

2018-06-22

In meromictic lakes, the water column is stratified into distinguishable steady layers with different physico-chemical properties. The bottom portion, known as monimolimnion, has been studied for the functional stratification of microbial populations. Recent experiments have reported the profiles of bacterial and nutrient spatial distributions, but quantitative understanding is invoked to unravel the underlying mechanism of maintaining the discrete spatial organization. Here a reaction-diffusion model is developed to highlight the spatial pattern coupled with the light-driven metabolism of bacteria, which is resilient to a wide range of dynamical correlation between bacterial and nutrient species at the molecular level. Particularly, exact analytical solutions of the system are presented together with numerical results, in a good agreement with measurements in Ace lake and Rogoznica lake. Furthermore, one quantitative prediction is reported here on the dynamics of the seasonal stratification patterns in Ace lake. The active role played by the bacterial metabolism at microscale clearly shapes the biogeochemistry landscape of lake-wide ecology at macroscale.

Numerical analysis of internal waves in stratified wake flows

NASA Astrophysics Data System (ADS)

Fraunie, Philppe

2014-05-01

In laboratory investigations, increased attention has been given to internal waves generated by stationary placed oscillating sources and moving bodies in stratified fluids [1]. The main attention was paid to study flows past bodies of perfect shapes like sphere [2], cylinder [3] of thin strip [3] which are the best theoretical (analytical or numerical) studies. Due to simplicity of geometry, flow around a strip has a potential to investigate separately effects of a drag and lift forces on the body by changing the slope of the horizontally moving strip which can be placed vertically [1], horizontally [2], or be tilted under some angle to the direction of towing velocity [5]. Numeric modeling of a flow past vertical strip uniformly towing with permanent velocity in horizontal direction in a linearly stratified talk which was based on a finite differences solver adapted to the low Reynolds Navier-Stokes equation with transport equation for salinity (LES simulation [6] and RANS [7]) has demonstrated reasonable agreement with data of Schlieren visualization, density marker and probe measurements of internal wave fields. The chosen test cases allowed demonstrating the ability of selected numerical methods to represent stably stratified flows over horizontal strip [4] and hill type 2D obstacles [1, 3] with generation of internal waves. ACKNOWLEDGMENTS This research work was supported by the Region Provence Alpes Côte d'Azur - Modtercom project. The work was also supported by the Russian Foundation for Basic Research (grant 12-01-00128). REFERENCES [1] Chashechkin Yu.D., Mitkin V.V. Experimental study of a fine structure of 2D wakes and mixing past an obstacle in a continuously stratified fluid // Dynamics of Atmosphere and Oceans. 2001. V. 34. P. 165-187. [2] Chashechkin, Yu. D. Hydrodynamics of a sphere in a stratified fluid // Fluid Dyn. 1989. V.24(1) P. 1-7. [3] Mitkin V. V., Chashechkin Yu. D. Transformation of hanging discontinuities into vortex systems in a stratified flow behind a cylinder // 2007. Fluid Dyn. V. 42 (1). P. 12-23. [4] Bardakov R. N., Mitkin V. V., Chashechkin Yu. D. Fine structure of a stratified flow near a flat-plate surface // J. Appl. Mech. Tech. Phys. 2007. V. 48(6) P. 840-851. [5] Chashechkin Yu. D., Mitkin V. V. An effect of a lift force on the structure of attached internal waves in a continuously stratified fluid // Dokl. Phys. 2001. V. 46 (6). P. 425-428. [6] Houcine H., Chashechkin Yu.D, Fraunié P., Fernando H.J.S., Gharbi A., Lili T. Numerical modeling of the generation of internal waves by uniform stratified flow over a thin vertical barrier // Int J. Num Methods in Fluids. 2012. V.68(4). P. 451-466. DOI: 10.1002/fld.2513 [7] Bodnar T., Benes , Fraunié P., Kozel K.. Application of Compact Finite-Difference Schemes to Simulations of Stably Stratified Fluid Flows. Applied Mathematics and Computation 219 : 3336-3353 2012. doi:10.1016/j.amc.2011.08.058

Secular rotational motions and the mechanical structure of a dynamical viscoelastic earth

NASA Technical Reports Server (NTRS)

Yuen, D. A.; Sabadini, R.

1984-01-01

A survey is presented of analytical methods for computing the linear responses of the rotational axis of a layered viscoelastic earth to surface loading. Theoretical research in this area is first summarized, and the differences between the mechanical boundary conditions to be applied at the interface separating the upper and lower mantles for an adiabatically and chemically stratified mantle are discussed. Some examples of polar wander and secular variation of the spin rate from glacial excitation are presented for various types of chemical and viscosity stratifications. The effects of an artificial density jump at the base of the lithosphere in models are examined, and certain issues concerning the fluid tidal Love number for different types of density stratification are addressed. The meaning of effective plate thickness over geological time scales for rotational dynamics is discussed.

Connecting macroscopic dynamics with microscopic properties in active microtubule network contraction

NASA Astrophysics Data System (ADS)

Foster, Peter J.; Yan, Wen; Fürthauer, Sebastian; Shelley, Michael J.; Needleman, Daniel J.

2017-12-01

The cellular cytoskeleton is an active material, driven out of equilibrium by molecular motor proteins. It is not understood how the collective behaviors of cytoskeletal networks emerge from the properties of the network’s constituent motor proteins and filaments. Here we present experimental results on networks of stabilized microtubules in Xenopus oocyte extracts, which undergo spontaneous bulk contraction driven by the motor protein dynein, and investigate the effects of varying the initial microtubule density and length distribution. We find that networks contract to a similar final density, irrespective of the length of microtubules or their initial density, but that the contraction timescale varies with the average microtubule length. To gain insight into why this microscopic property influences the macroscopic network contraction time, we developed simulations where microtubules and motors are explicitly represented. The simulations qualitatively recapitulate the variation of contraction timescale with microtubule length, and allowed stress contributions from different sources to be estimated and decoupled.

Intermittent gravity-driven flow of grains through narrow pipes

NASA Astrophysics Data System (ADS)

Alvarez, Carlos A.; de Moraes Franklin, Erick

2017-01-01

Grain flows through pipes are frequently found in various settings, such as in pharmaceutical, chemical, petroleum, mining and food industries. In the case of size-constrained gravitational flows, density waves consisting of alternating high- and low-compactness regions may appear. This study investigates experimentally the dynamics of density waves that appear in gravitational flows of fine grains through vertical and slightly inclined pipes. The experimental device consisted of a transparent glass pipe through which different populations of glass spheres flowed driven by gravity. Our experiments were performed under controlled ambient temperature and relative humidity, and the granular flow was filmed with a high-speed camera. Experimental results concerning the length scales and celerities of density waves are presented, together with a one-dimensional model and a linear stability analysis. The analysis exhibits the presence of a long-wavelength instability, with the most unstable mode and a cut-off wavenumber whose values are in agreement with the experimental results.

Impacts of four decades of stand density management treatments on wood properties of loblolly pine

Treesearch

M.A. Blazier; A. Clark; J.M. Mahon; M.R. Strub; R.F. Daniels; L.R. Schimleck

2013-01-01

Stand density management is a powerful silvicultural tool for manipulating stand volumes, but it has the potential to alter key wood properties. At a site in northcentral Louisiana, five density management regimes were conducted over a 45-year period. At age 49, a stratified sample of trees was destructively harvested for crown length, taper, and specific gravity...

Comparison between numerical and analytical results on the required rf current for stabilizing neoclassical tearing modes

NASA Astrophysics Data System (ADS)

Wang, Xiaojing; Yu, Qingquan; Zhang, Xiaodong; Zhang, Yang; Zhu, Sizheng; Wang, Xiaoguang; Wu, Bin

2018-04-01

Numerical studies on the stabilization of neoclassical tearing modes (NTMs) by electron cyclotron current drive (ECCD) have been carried out based on reduced MHD equations, focusing on the amount of the required driven current for mode stabilization and the comparison with analytical results. The dependence of the minimum driven current required for NTM stabilization on some parameters, including the bootstrap current density, radial width of the driven current, radial deviation of the driven current from the resonant surface, and the island width when applying ECCD, are studied. By fitting the numerical results, simple expressions for these dependences are obtained. Analysis based on the modified Rutherford equation (MRE) has also been carried out, and the corresponding results have the same trend as numerical ones, while a quantitative difference between them exists. This difference becomes smaller when the applied radio frequency (rf) current is smaller.

Effects of initial condition spectral content on shock-driven turbulent mixing.

PubMed

Nelson, Nicholas J; Grinstein, Fernando F

2015-07-01

The mixing of materials due to the Richtmyer-Meshkov instability and the ensuing turbulent behavior is of intense interest in a variety of physical systems including inertial confinement fusion, combustion, and the final stages of stellar evolution. Extensive numerical and laboratory studies of shock-driven mixing have demonstrated the rich behavior associated with the onset of turbulence due to the shocks. Here we report on progress in understanding shock-driven mixing at interfaces between fluids of differing densities through three-dimensional (3D) numerical simulations using the rage code in the implicit large eddy simulation context. We consider a shock-tube configuration with a band of high density gas (SF(6)) embedded in low density gas (air). Shocks with a Mach number of 1.26 are passed through SF(6) bands, resulting in transition to turbulence driven by the Richtmyer-Meshkov instability. The system is followed as a rarefaction wave and a reflected secondary shock from the back wall pass through the SF(6) band. We apply a variety of initial perturbations to the interfaces between the two fluids in which the physical standard deviation, wave number range, and the spectral slope of the perturbations are held constant, but the number of modes initially present is varied. By thus decreasing the density of initial spectral modes of the interface, we find that we can achieve as much as 25% less total mixing at late times. This has potential direct implications for the treatment of initial conditions applied to material interfaces in both 3D and reduced dimensionality simulation models.

Effects of Initial Condition Spectral Content on Shock Driven-Turbulent Mixing

DOE PAGES

Nelson, Nicholas James; Grinstein, Fernando F.

2015-07-15

The mixing of materials due to the Richtmyer-Meshkov instability and the ensuing turbulent behavior is of intense interest in a variety of physical systems including inertial confinement fusion, combustion, and the final stages of stellar evolution. Extensive numerical and laboratory studies of shock-driven mixing have demonstrated the rich behavior associated with the onset of turbulence due to the shocks. Here we report on progress in understanding shock-driven mixing at interfaces between fluids of differing densities through three-dimensional (3D) numerical simulations using the RAGE code in the implicit large eddy simulation context. We consider a shock-tube configuration with a band ofmore » high density gas (SF 6) embedded in low density gas (air). Shocks with a Mach number of 1.26 are passed through SF 6 bands, resulting in transition to turbulence driven by the Richtmyer-Meshkov instability. The system is followed as a rarefaction wave and a reflected secondary shock from the back wall pass through the SF 6 band. We apply a variety of initial perturbations to the interfaces between the two fluids in which the physical standard deviation, wave number range, and the spectral slope of the perturbations are held constant, but the number of modes initially present is varied. By thus decreasing the density of initial spectral modes of the interface, we find that we can achieve as much as 25% less total mixing at late times. This has potential direct implications for the treatment of initial conditions applied to material interfaces in both 3D and reduced dimensionality simulation models.« less

DNS of helicity-induced stratified turbulent flow

NASA Astrophysics Data System (ADS)

Chandy, Abhilash J.; Rahimi, Abbas

2013-11-01

Helical flows undergoing density stratification have wide applications in meteorological phenomena such as dust devils, tornadoes, and hurricanes due to the complexity and disasters caused by them. Direct numerical simulations (DNS) of transition to turbulence in a stably stratified Boussinesq fluid are presented for different rotation and stratification intensities. In order to understand the effect of velocity on the energy cascade, comparisons are made between helicity initiated and non-helical flows. Results show that stratification decelerates the helicity decay and causes velocity and vorticity to align with each other. With respect to the helical and non-helical flow comparisons, the total energy in the presence of stratification decays faster with helicity. In addition, the behavior of length scales were examined by comparing temporal variations of the vertical shearing of velocities. Results showed a growing asymmetry with time in the case of helical flow, while non-helical flow stayed close to begin symmetric.

A Geophysical Flow Experiment in a Compressible Critical Fluid

NASA Technical Reports Server (NTRS)

Hegseth, John; Garcia, Laudelino

1996-01-01

The first objective of this experiment is to build an experimental system in which, in analogy to a geophysical system, a compressible fluid in a spherical annulus becomes radially stratified in density through an A.C. electric field. When this density gradient is demonstrated, the system will be augmented so that the fluid can be driven by heating and rotation and tested in preparation for a microgravity experiment. This apparatus consists of a spherical capacitor filled with critical fluid in a temperature controlled environment. To make the fluid critical, the apparatus will be operated near the critical pressure, critical density, and critical temperature of the fluid. This will result in a highly compressible fluid because of the properties of the fluid near its critical point. A high voltage A.C. source applied across the capacitor will create a spherically symmetric central force because of the dielectric properties of the fluid in an electric field gradient. This central force will induce a spherically symmetric density gradient that is analogous to a geophysical fluid system. To generate such a density gradient the system must be small (approx. 1 inch diameter). This small cell will also be capable of driving the critical fluid by heating and rotation. Since a spherically symmetric density gradient can only be made in microgravity, another small cell, of the same geometry, will be built that uses incompressible fluid. The driving of the fluid by rotation and heating in these small cells will be developed. The resulting instabilities from the driving in these two systems will then be studied. The second objective is to study the pattern forming instabilities (bifurcations) resulting from the well controlled experimental conditions in the critical fluid cell. This experiment will come close to producing conditions that are geophysically similar and will be studied as the driving parameters are changed.

Phreatomagmatic eruptive and depositional processes during the 1949 eruption on La Palma (Canary Islands)

NASA Astrophysics Data System (ADS)

White, James D. L.; Schmincke, Hans-Ulrich

1999-12-01

In 1949, a 5-week-long magmatic and phreatomagmatic eruption took place along the active volcanic ridge of La Palma (Canary Islands). Two vents, Duraznero and Hoyo Negro, produced significant pyroclastic deposits. The eruption began from Duraznero vent, which produced a series of deposits with an upward decrease in accidental fragments and increase in fluidal ash and spatter, together inferred to indicate decreasing phreatomagmatic interaction. Hoyo Negro erupted over a 2-week period, producing a variety of pyroclastic density currents and ballistic blocks and bombs. Hoyo Negro erupted within and modified an older crater having high walls on the northern to southeastern edges. Southwestern to western margins of the crater lay 50 to 100 m lower. Strongly contrasting deposits in the different sectors (N-SE vs. SW-W) were formed as a result of interaction between topography, weak eruptive columns and stratified pyroclastic density currents. Tephra ring deposits are thicker and coarser-grained than upper rim deposits formed along the higher edges of the crater, and beyond the crater margin, valley-confined deposits are thicker than more thinly bedded mantling deposits on higher topography. These differences indicate that the impact zone for the bulk of the collapsing, tephra-laden column lay within the crater and that the high crater walls inhibited escape of pyroclastic density currents to the north and east. The impact zone lay outside the low SW-W rims, however, thus allowing stratified pyroclastic density currents to move freely away from the crater in those directions, depositing thin sections (<30 cm) of well-bedded ash (mantling deposits) on ridges and thicker sections (1-3 m) of structureless ash beds in valleys and small basins. Such segregation of dense pyroclastic currents from more dilute ones at the crater wall is likely to be common for small eruptions from pre-existing craters and is an important factor to be taken into account in volcanic hazards assessments.

Jet-mixing of initially-stratified liquid-liquid pipe flows: experiments and numerical simulations

NASA Astrophysics Data System (ADS)

Wright, Stuart; Ibarra-Hernandes, Roberto; Xie, Zhihua; Markides, Christos; Matar, Omar

2016-11-01

Low pipeline velocities lead to stratification and so-called 'phase slip' in horizontal liquid-liquid flows due to differences in liquid densities and viscosities. Stratified flows have no suitable single point for sampling, from which average phase properties (e.g. fractions) can be established. Inline mixing, achieved by static mixers or jets in cross-flow (JICF), is often used to overcome liquid-liquid stratification by establishing unstable two-phase dispersions for sampling. Achieving dispersions in liquid-liquid pipeline flows using JICF is the subject of this experimental and modelling work. The experimental facility involves a matched refractive index liquid-liquid-solid system, featuring an ETFE test section, and experimental liquids which are silicone oil and a 51-wt% glycerol solution. The matching then allows the dispersed fluid phase fractions and velocity fields to be established through advanced optical techniques, namely PLIF (for phase) and PTV or PIV (for velocity fields). CFD codes using the volume of a fluid (VOF) method are then used to demonstrate JICF breakup and dispersion in stratified pipeline flows. A number of simple jet configurations are described and their dispersion effectiveness is compared with the experimental results. Funding from Cameron for Ph.D. studentship (SW) gratefully acknowledged.

Water Quality Research Program. Simultaneous, Multiple-Level Withdrawal from a Density Stratified Reservoir

DTIC Science & Technology

1990-11-01

Furusawa , M . 1969. "Experiments on Selective Withdrawal into a Sink from a Uniformly Stratified Fluid--Phenomenon of Layer Separation and Middle-Layer...E. Howington prepared this report. Assisting in the testing were Messrs. Jack E. Davis; Calvin Buie, Jr.; Douglas M . White; and Paul Ahlrich; and Ms...virtually horizontal. Several researchers such as Hino and Furusawa (1969), Croach (1971), Hino (1980), and Farrant (1982) have studied withdrawal

Relation between body composition and bone mineral density in young undregraduate students with different nutritional status

PubMed Central

Rodrigues, Edil de Albuquerque; dos Santos, Marcos André Moura; da Silva, Amanda Tabosa Pereira; Farah, Breno Quintella; Costa, Manoel da Cunha; Campos, Florisbela de Arruda Camara e Siqueira; Falcão, Ana Patrícia Siqueira Tavares

2016-01-01

ABSTRACT Objective To investigate the relationship between total and segmental body fat, bone mineral density and bone mineral content in undergraduate students stratified according to nutritional status. Methods The study included 45 male undergraduate students aged between 20 and 30 years. Total and segmental body composition, bone mineral density and bone mineral content assessments were performed using dual energy X-ray absorptiometry. Subjects were allocated into three groups (eutrophic, overweight and obese). Results With the exception of upper limb bone mineral content, significantly higher (p<0.05) mean bone mineral density, bone mineral content, and relative body fat values were documented in the obese group. Total body and segmental relative body fat (lower limbs and trunk) were positively correlated (p<0.05) with bone mineral density in the overweight group. Upper limb fat was negatively correlated (p<0.05) with bone mineral content in the normal and eutrophic groups. Conclusion Total body and segmental body fat were correlated with bone mineral density and bone mineral content in male undergraduate students, particularly in overweight individuals. PMID:27074228

A phenomenological continuum model for force-driven nano-channel liquid flows

NASA Astrophysics Data System (ADS)

Ghorbanian, Jafar; Celebi, Alper T.; Beskok, Ali

2016-11-01

A phenomenological continuum model is developed using systematic molecular dynamics (MD) simulations of force-driven liquid argon flows confined in gold nano-channels at a fixed thermodynamic state. Well known density layering near the walls leads to the definition of an effective channel height and a density deficit parameter. While the former defines the slip-plane, the latter parameter relates channel averaged density with the desired thermodynamic state value. Definitions of these new parameters require a single MD simulation performed for a specific liquid-solid pair at the desired thermodynamic state and used for calibration of model parameters. Combined with our observations of constant slip-length and kinematic viscosity, the model accurately predicts the velocity distribution and volumetric and mass flow rates for force-driven liquid flows in different height nano-channels. Model is verified for liquid argon flow at distinct thermodynamic states and using various argon-gold interaction strengths. Further verification is performed for water flow in silica and gold nano-channels, exhibiting slip lengths of 1.2 nm and 15.5 nm, respectively. Excellent agreements between the model and the MD simulations are reported for channel heights as small as 3 nm for various liquid-solid pairs.

Perspectives on continuum flow models for force-driven nano-channel liquid flows

NASA Astrophysics Data System (ADS)

Beskok, Ali; Ghorbanian, Jafar; Celebi, Alper

2017-11-01

A phenomenological continuum model is developed using systematic molecular dynamics (MD) simulations of force-driven liquid argon flows confined in gold nano-channels at a fixed thermodynamic state. Well known density layering near the walls leads to the definition of an effective channel height and a density deficit parameter. While the former defines the slip-plane, the latter parameter relates channel averaged density with the desired thermodynamic state value. Definitions of these new parameters require a single MD simulation performed for a specific liquid-solid pair at the desired thermodynamic state and used for calibration of model parameters. Combined with our observations of constant slip-length and kinematic viscosity, the model accurately predicts the velocity distribution and volumetric and mass flow rates for force-driven liquid flows in different height nano-channels. Model is verified for liquid argon flow at distinct thermodynamic states and using various argon-gold interaction strengths. Further verification is performed for water flow in silica and gold nano-channels, exhibiting slip lengths of 1.2 nm and 15.5 nm, respectively. Excellent agreements between the model and the MD simulations are reported for channel heights as small as 3 nm for various liquid-solid pairs.

Experimental and Numerical Investigation of Internal Gravity Waves Excited by Turbulent Penetrative Convection in Water Around Its Density Maximum

NASA Astrophysics Data System (ADS)

Perrard, Stéphane; Le Bars, Michaël; Le Gal, Patrice

This study is devoted to the experimental and numerical analysis of the excitation of gravity waves by turbulent convection. This situation is representative of many geophysical or astrophysical systems such as the convective bottom layer of the atmosphere that radiates internal waves in the stratosphere, or the interaction between the convective and the radiative zones in stars. In our experiments, we use water as a working fluid as it possesses the remarkable property of having a maximum density at 4 °C. Therefore, when establishing on a water layer a temperature gradient between 0 °C at the bottom and room temperature at the top, a turbulent convective region appears spontaneously under a stably stratified zone. In these conditions, gravity waves are excited by the convective fluid motions penetrating the stratified layer. Although this type of flow, called penetrative convection, has already been described, we present here the first velocity field measurement of wave emission and propagation. We show in particular that an intermediate layer that we call the buffer layer emerges between the convective and the stratified zones. In this buffer layer, the angle of propagation of the waves varies with the altitude since it is slaved to the Brunt-Väisälä frequency which evolves rapidly between the convective and the stratified layer. A minimum angle is reached at the end of the buffer layer. Then we observe that an angle of propagation is selected when the waves travel through the stratified layer. We expect this process of wave selection to take place in natural situations.

Modelling the distribution of domestic ducks in Monsoon Asia

USGS Publications Warehouse

Van Bockel, Thomas P.; Prosser, Diann; Franceschini, Gianluca; Biradar, Chandra; Wint, William; Robinson, Tim; Gilbert, Marius

2011-01-01

Domestic ducks are considered to be an important reservoir of highly pathogenic avian influenza (HPAI), as shown by a number of geospatial studies in which they have been identified as a significant risk factor associated with disease presence. Despite their importance in HPAI epidemiology, their large-scale distribution in Monsoon Asia is poorly understood. In this study, we created a spatial database of domestic duck census data in Asia and used it to train statistical distribution models for domestic duck distributions at a spatial resolution of 1km. The method was based on a modelling framework used by the Food and Agriculture Organisation to produce the Gridded Livestock of the World (GLW) database, and relies on stratified regression models between domestic duck densities and a set of agro-ecological explanatory variables. We evaluated different ways of stratifying the analysis and of combining the prediction to optimize the goodness of fit of the predictions. We found that domestic duck density could be predicted with reasonable accuracy (mean RMSE and correlation coefficient between log-transformed observed and predicted densities being 0.58 and 0.80, respectively), using a stratification based on livestock production systems. We tested the use of artificially degraded data on duck distributions in Thailand and Vietnam as training data, and compared the modelled outputs with the original high-resolution data. This showed, for these two countries at least, that these approaches could be used to accurately disaggregate provincial level (administrative level 1) statistical data to provide high resolution model distributions.

Ergodicity-breaking bifurcations and tunneling in hyperbolic transport models

NASA Astrophysics Data System (ADS)

Giona, M.; Brasiello, A.; Crescitelli, S.

2015-11-01

One of the main differences between parabolic transport, associated with Langevin equations driven by Wiener processes, and hyperbolic models related to generalized Kac equations driven by Poisson processes, is the occurrence in the latter of multiple stable invariant densities (Frobenius multiplicity) in certain regions of the parameter space. This phenomenon is associated with the occurrence in linear hyperbolic balance equations of a typical bifurcation, referred to as the ergodicity-breaking bifurcation, the properties of which are thoroughly analyzed.

Cosmic ray driven outflows in an ultraluminous galaxy

NASA Astrophysics Data System (ADS)

Fujita, Akimi; Mac Low, Mordecai-Mark

2018-06-01

In models of galaxy formation, feedback driven both by supernova (SN) and active galactic nucleus is not efficient enough to quench star formation in massive galaxies. Models of smaller galaxies have suggested that cosmic rays (CRs) play a major role in expelling material from the star-forming regions by diffusing SN energy to the lower density outskirts. We therefore run gas dynamical simulations of galactic outflows from a galaxy contained in a halo with 5 × 1012 M⊙ that resembles a local ultraluminous galaxy, including both SN thermal energy and a treatment of CRs using the same diffusion approximation as Salem & Bryan. We find that CR pressure drives a low-density bubble beyond the edge of the shell swept up by thermal pressure, but the main bubble driven by SN thermal pressure overtakes it later, which creates a large-scale biconical outflow. CRs diffusing into the disc are unable to entrain its gas in the outflows, yielding a mass-loading rate of only ˜ 0.1 per cent with varied CR diffusion coefficients. We find no significant difference in mass-loading rates in SN-driven outflows with or without CR pressure. Our simulations strongly suggest that it is hard to drive a heavily mass-loaded outflow with CRs from a massive halo potential, although more distributed star formation could lead to a different result.

Experimental study of mixing mechanisms in stably stratified Taylor-Couette flow

NASA Astrophysics Data System (ADS)

Augier, Pierre; Caulfield, Colm-Cille; Dalziel, Stuart

2014-11-01

We consider experimentally the mechanisms of mixing in stably stratified Taylor-Couette (TC) flow in a TC apparatus for which both cylinders can rotate independently. In the case for which only the inner cylinder rotates, centrifugal instability rapidly splits an initially linear density profile into an array of thin nearly homogeneous layers. Shadowgraph, PIV and density profiles measured by a moving conductivity probe allow us to characterise this process and the resulting flow. In particular, we observe turbulent intrusions of mixed fluid propagating relatively slowly around the tank at the interfaces between the layers, leading to a time-dependent variation in the sharpness and turbulent activity at these interfaces, whose period scales with (but is much larger than) the rotation period. Interestingly, the turbulent intrusions are anti-correlated between adjacent interfaces leading to snake-skin-like patterns in the spatio-temporal diagrams of the density profiles. We also explore how the presence of a density stratification modifies end effects at the top and bottom of the cylinders, in both the presence and absence of primary centrifugal instability.

Artificial cognitive memory—changing from density driven to functionality driven

NASA Astrophysics Data System (ADS)

Shi, L. P.; Yi, K. J.; Ramanathan, K.; Zhao, R.; Ning, N.; Ding, D.; Chong, T. C.

2011-03-01

Increasing density based on bit size reduction is currently a main driving force for the development of data storage technologies. However, it is expected that all of the current available storage technologies might approach their physical limits in around 15 to 20 years due to miniaturization. To further advance the storage technologies, it is required to explore a new development trend that is different from density driven. One possible direction is to derive insights from biological counterparts. Unlike physical memories that have a single function of data storage, human memory is versatile. It contributes to functions of data storage, information processing, and most importantly, cognitive functions such as adaptation, learning, perception, knowledge generation, etc. In this paper, a brief review of current data storage technologies are presented, followed by discussions of future storage technology development trend. We expect that the driving force will evolve from density to functionality, and new memory modules associated with additional functions other than only data storage will appear. As an initial step toward building a future generation memory technology, we propose Artificial Cognitive Memory (ACM), a memory based intelligent system. We also present the characteristics of ACM, new technologies that can be used to develop ACM components such as bioinspired element cells (silicon, memristor, phase change, etc.), and possible methodologies to construct a biologically inspired hierarchical system.

Dissipation-induced dipole blockade and antiblockade in driven Rydberg systems

NASA Astrophysics Data System (ADS)

Young, Jeremy T.; Boulier, Thomas; Magnan, Eric; Goldschmidt, Elizabeth A.; Wilson, Ryan M.; Rolston, Steven L.; Porto, James V.; Gorshkov, Alexey V.

2018-02-01

We study theoretically and experimentally the competing blockade and antiblockade effects induced by spontaneously generated contaminant Rydberg atoms in driven Rydberg systems. These contaminant atoms provide a source of strong dipole-dipole interactions and play a crucial role in the system's behavior. We study this problem theoretically using two different approaches. The first is a cumulant expansion approximation, in which we ignore third-order and higher connected correlations. Using this approach for the case of resonant drive, a many-body blockade radius picture arises, and we find qualitative agreement with previous experimental results. We further predict that as the atomic density is increased, the Rydberg population's dependence on Rabi frequency will transition from quadratic to linear dependence at lower Rabi frequencies. We study this behavior experimentally by observing this crossover at two different atomic densities. We confirm that the larger density system has a smaller crossover Rabi frequency than the smaller density system. The second theoretical approach is a set of phenomenological inhomogeneous rate equations. We compare the results of our rate-equation model to the experimental observations [E. A. Goldschmidt et al., Phys. Rev. Lett. 116, 113001 (2016), 10.1103/PhysRevLett.116.113001] and find that these rate equations provide quantitatively good scaling behavior of the steady-state Rydberg population for both resonant and off-resonant drives.

Dissipation in the Baltic proper during winter stratification

NASA Astrophysics Data System (ADS)

Lass, Hans Ulrich; Prandke, Hartmut; Liljebladh, Bengt

2003-06-01

Profiles of dissipation rates and stratification between 10 and 120 m depth were measured with a loosely tethered profiler over a 9-day winter period in the Gotland Basin of the Baltic Sea. Supplementary measurements of current profiles were made with moored ADCPs. Temporal and spatial patterns of the stratification were observed by means of towed CTD. Shallow freshwater lenses in the surface mixed layer, mesoscale eddies, inertial oscillations, and inertial waves as part of the internal wave spectrum provided the marine physical environment for the small-scale turbulence. Two well-separated turbulence regimes were detected. The turbulence in the surface mixed layer was well correlated with the wind. The majority of the energy flux from the wind to the turbulent kinetic energy was dissipated within the surface mixed layer. A minor part of this flux was consumed by changes of the potential energy of the fresh water lenses. The penetration depth Hpen of the wind-driven turbulence into the weakly stratified surface mixed layer depended on the local wind speed (W10) as Hpen = cW103/2 Active erosion of the Baltic halocline by wind-driven turbulence is expected for wind speeds greater than 14 m/s. The turbulence in the strongly stratified interior of the water column was quite independent of the meteorological forcing at the sea surface. The integrated production of turbulent kinetic energy exceeded the energy loss of inertial oscillations in the surface layer suggesting additional energy sources which might have been provided by inertial wave radiation during geostrophic adjustment of coastal jets and mesoscale eddies. The averaged dissipation rate profile in the stratified part of the water column, best fitted by ɛ ∝ EN, was different from the scaling of the dissipation in the thermocline of the ocean [, 1986]. The diapycnical mixing coefficient (Kv) was best fit by Kv = a0/N according to [1987] with a0 ≈ 0.87 × 10-7 m2/s2. The diapycnal diffusivity estimated from the dissipation rate was lower than those estimated by the bulk method.

Effect of small-scale turbulence on feeding rates of larval cod and haddock in stratified water on Georges Bank

NASA Astrophysics Data System (ADS)

Gregory Lough, R.; Mountain, David G.

A set of vertically stratified MOCNESS tows made on the southern flank of Georges Bank in spring 1981 and 1983 was analyzed to examine the relationship between larval cod and haddock feeding success and turbulent dissipation in a stratified water column. Observed feeding ratios (mean no. prey larval gut -1) for three size classes of larvae were compared with estimated ingestion rates using the Rothschild and Osborn ( Journal of Plankton Research, 10, 1988, 465-474) predator-prey encounter rate model. Simulation of contact rates requires parameter estimates of larval fish and their prey cruising speeds, density of prey, and turbulent velocity of the water column. Turbulent dissipation was estimated from a formulation by James ( Estuarine and Coastal Marine Science, 5, 1977, 339-353) incorporating both a wind a tidal component. Larval ingestion rates were based on swallowing probabilities derived from calm-water laboratory observations. Model-predicted turbulence profiles generally showed that dissipation rates were low to moderate (10 -11-10 -7 W kg -1). Turbulence was minimal at or below the pycnocline (≈ 25 m) with higher values(1-2 orders of magnitude) near the surface due to wind mixing and at depth due to shear in the tidal current near bottom. In a stratified water column during the day, first-feeding larvae (5-6 mm) were located mostly within or above the pycnocline coincident with their copepod prey (nauplii and copepodites). The 7-8 mm larvae were most abundant within the pycnocline, whereas the 9-10 mm larvae were found within and below the pycnocline. Feeding ratios were relatively low in early morning following darkness when the wind speed was low, but increased by a factor of 2-13 by noon and evening when the wind speed doubled. Comparison of depth-specific feeding ratios with estimated ingestion rates, derived from turbulence-affected contact rates, generally were reasonable after allowing for an average gut evacuation time (4 h), and in many cases the observed and estimated values had similar profiles. However, differences in vertical profiles may be attributed to differential digestion time, pursuit behavior affected by high turbulence, vertical migration of the larger larvae, an optimum light level for feeding, smaller-scale prey patchiness, and the gross estimates of turbulence. Response-surface estimation of averaged feeding ratios as a function of averaged prey density (0-50 m) with a minimum water-column turbulence value predicted that 5-6 mm larvae have a maximum feeding response at the highest prey densities (> 30 prey 1 -1) and lower turbulence estimates (<10 -10 W kg -1). The 7-8 mm and 9-10 mm larvae also have a maximum feeding response at high prey densities and low turbulence, but it extends to lower prey densities (> 10 prey 1 -1) as turbulence increases to intermidiate levels, clearly showing an interaction effect. In general, maximum feeding ratios occur at low to intermediate levels of turbulence where average prey density is greater than 10-20 prey 1 -1.

Unique determination of stratified steady water waves from pressure

NASA Astrophysics Data System (ADS)

Chen, Robin Ming; Walsh, Samuel

2018-01-01

Consider a two-dimensional stratified solitary wave propagating through a body of water that is bounded below by an impermeable ocean bed. In this work, we study how such a wave can be recovered from data consisting of the wave speed, upstream and downstream density and velocity profile, and the trace of the pressure on the bed. In particular, we prove that this data uniquely determines the wave, both in the (real) analytic and Sobolev regimes.

Sex-related differences in sleep slow wave activity in major depressive disorder: a high-density EEG investigation.

PubMed

Plante, David T; Landsness, Eric C; Peterson, Michael J; Goldstein, Michael R; Riedner, Brady A; Wanger, Timothy; Guokas, Jeffrey J; Tononi, Giulio; Benca, Ruth M

2012-09-18

Sleep disturbance plays an important role in major depressive disorder (MDD). Prior investigations have demonstrated that slow wave activity (SWA) during sleep is altered in MDD; however, results have not been consistent across studies, which may be due in part to sex-related differences in SWA and/or limited spatial resolution of spectral analyses. This study sought to characterize SWA in MDD utilizing high-density electroencephalography (hdEEG) to examine the topography of SWA across the cortex in MDD, as well as sex-related variation in SWA topography in the disorder. All-night recordings with 256 channel hdEEG were collected in 30 unipolar MDD subjects (19 women) and 30 age and sex-matched control subjects. Spectral analyses of SWA were performed to determine group differences. SWA was compared between MDD and controls, including analyses stratified by sex, using statistical non-parametric mapping to correct for multiple comparisons of topographic data. As a group, MDD subjects demonstrated significant increases in all-night SWA primarily in bilateral prefrontal channels. When stratified by sex, MDD women demonstrated global increases in SWA relative to age-matched controls that were most consistent in bilateral prefrontal regions; however, MDD men showed no significant differences relative to age-matched controls. Further analyses demonstrated increased SWA in MDD women was most prominent in the first portion of the night. Women, but not men with MDD demonstrate significant increases in SWA in multiple cortical areas relative to control subjects. Further research is warranted to investigate the role of SWA in MDD, and to clarify how increased SWA in women with MDD is related to the pathophysiology of the disorder.

Experimental study of a fine structure of 2D wakes and mixing past an obstacle in a continuously stratified fluid

NASA Astrophysics Data System (ADS)

Chashechkin, Yuli. D.; Mitkin, Vladimir V.

2001-10-01

Experimental investigations of fine and macroscopic structures of density and velocity disturbances generated by a towing cylinder or a vertical strip in a linearly stratified liquid are carried out in a rectangular tank. A density gradient field is visualised by different Schlieren methods (direct shadow, 'slit-knife', 'slit-thread', 'natural rainbow') characterised by a high spatial resolution. Profiles of fluid velocity are visualised by density markers — wakes past a vertically descending sugar crystal or an ascending gas bubble. In a fluid at rest, the density marker acts as a vertical linear source of internal oscillations which allows us to measure buoyancy frequency over all depth by the Schlieren instrument directly or by a conductivity probe in a particular point. Sensitive methods reveal a set of high gradient interfaces inside and outside the downstream wake besides well-known large scale elements: upstream disturbances, attached internal waves and vortices. Solitary interfaces located inside the attached internal waves field have no features on their leading and trailing edges. A thickness of interfaces is defined by an appropriate diffusion coefficient and a buoyancy frequency. High gradient interfaces bound compact vortices. Vortices moving with respect to environment emit their own systems of internal waves randomising a regular pattern of attached antisymmetric internal waves. But after a rather long time a wave recurrence occurs and a regular but symmetric structure of the longest waves (similar to the pattern of initial attached internal waves) is observed again. High gradient interfaces and lines of their intersections act as collectors of a dye coming from a compact source or from a coloured liquid volume inside the tank and separate coloured and clear areas.

On the Rigid-Lid Approximation for Two Shallow Layers of Immiscible Fluids with Small Density Contrast

NASA Astrophysics Data System (ADS)

Duchêne, Vincent

2014-08-01

The rigid-lid approximation is a commonly used simplification in the study of density-stratified fluids in oceanography. Roughly speaking, one assumes that the displacements of the surface are negligible compared with interface displacements. In this paper, we offer a rigorous justification of this approximation in the case of two shallow layers of immiscible fluids with constant and quasi-equal mass density. More precisely, we control the difference between the solutions of the Cauchy problem predicted by the shallow-water (Saint-Venant) system in the rigid-lid and free-surface configuration. We show that in the limit of a small density contrast, the flow may be accurately described as the superposition of a baroclinic (or slow) mode, which is well predicted by the rigid-lid approximation, and a barotropic (or fast) mode, whose initial smallness persists for large time. We also describe explicitly the first-order behavior of the deformation of the surface and discuss the case of a nonsmall initial barotropic mode.

Layering of sustained vortices in rotating stratified fluids

NASA Astrophysics Data System (ADS)

Aubert, O.; Le Bars, M.; Le Gal, P.

2013-05-01

The ocean is a natural stratified fluid layer where large structures are influenced by the rotation of the planet through the Coriolis force. In particular, the ocean Meddies are long-lived anticyclonic pancake vortices of Mediterranean origin evolving in the Atlantic Ocean: they have a saltier and warmer core than the sourrounding oceanic water, their diameters go up to 100 km and they can survive for 2 to 3 years in the ocean. Their extensive study using seismic images revealed finestructures surrounding their core (Biescas et al., 2008; Ruddick et al., 2009) corresponding to layers of constant density which thickness is about 40 m and horizontal extent is more than 10 km. These layers can have different origins: salt fingers from a double-diffusive instabilities of salt and heat (Ruddick & Gargett, 2003), viscous overturning motions from a double-diffusive instabilities of salt and momentum (McIntyre, 1970) or global modes of the quasi-geostrophic instability (Nguyen et al., 2011)? As observed by Griffiths & Linden (1981), sustained laboratory anticyclonic vortices created via a continuous injection of isodense fluid in a rotating and linearly stratified layer of salty water are quickly surrounded by layers of constant density. In the continuity of their experiments, we systematically investigated the double-diffusive instability of McIntyre by varying the Coriolis parameter f and the buoyancy frequency N of the background both in experiments and in numerical simulations, and studied the influence of the Schmidt number in numerical simulations. Following McIntyre's approach, typical length and time scales of the instability are well described by a linear stability analysis based on a gaussian model that fits both laboratory and oceanic vortices. The instability appears to be favoured by high Rossby numbers and ratios f/N. We then apply these results to ocean Meddies and conclude about their stability.

Buoyant miscible displacement flow of shear-thinning fluids: Experiments and Simulations

NASA Astrophysics Data System (ADS)

Ale Etrati Khosroshahi, Seyed Ali; Frigaard, Ian

2017-11-01

We study displacement flow of two miscible fluids with density and viscosity contrast in an inclined pipe. Our focus is mainly on displacements where transverse mixing is not significant and thus a two-layer, stratified flow develops. Our experiments are carried out in a long pipe, covering a wide range of flow-rates, inclination angles and viscosity ratios. Density and viscosity contrasts are achieved by adding Glycerol and Xanthan gum to water, respectively. At each angle, flow rate and viscosity ratio are varied and density contrast is fixed. We identify and map different flow regimes, instabilities and front dynamics based on Fr , Re / Frcosβ and viscosity ratio m. The problem is also studied numerically to get a better insight into the flow structure and shear-thinning effects. Numerical simulations are completed using OpenFOAM in both pipe and channel geometries and are compared against the experiments. Schlumberger, NSERC.

Intraspecific Relationships among Wood Density, Leaf Structural Traits and Environment in Four Co-Occurring Species of Nothofagus in New Zealand

PubMed Central

Richardson, Sarah J.; Allen, Robert B.; Buxton, Rowan P.; Easdale, Tomás A.; Hurst, Jennifer M.; Morse, Christopher W.; Smissen, Rob D.; Peltzer, Duane A.

2013-01-01

Plant functional traits capture important variation in plant strategy and function. Recent literature has revealed that within-species variation in traits is greater than previously supposed. However, we still have a poor understanding of how intraspecific variation is coordinated among different traits, and how it is driven by environment. We quantified intraspecific variation in wood density and five leaf traits underpinning the leaf economics spectrum (leaf dry matter content, leaf mass per unit area, size, thickness and density) within and among four widespread Nothofagus tree species in southern New Zealand. We tested whether intraspecific relationships between wood density and leaf traits followed widely reported interspecific relationships, and whether variation in these traits was coordinated through shared responses to environmental factors. Sample sites varied widely in environmental variables, including soil fertility (25–900 mg kg–1 total P), precipitation (668–4875 mm yr–1), temperature (5.2–12.4 °C mean annual temperature) and latitude (41–46 °S). Leaf traits were strongly correlated with one another within species, but not with wood density. There was some evidence for a positive relationship between wood density and leaf tissue density and dry matter content, but no evidence that leaf mass or leaf size were correlated with wood density; this highlights that leaf mass per unit area cannot be used as a surrogate for component leaf traits such as tissue density. Trait variation was predicted by environmental factors, but not consistently among different traits; e.g., only leaf thickness and leaf density responded to the same environmental cues as wood density. We conclude that although intraspecific variation in wood density and leaf traits is strongly driven by environmental factors, these responses are not strongly coordinated among functional traits even across co-occurring, closely-related plant species. PMID:23527041

Modeling the seasonal circulation in Massachusetts Bay

USGS Publications Warehouse

Signell, Richard P.; Jenter, Harry L.; Blumberg, Alan F.; ,

1994-01-01

An 18 month simulation of circulation was conducted in Massachusetts Bay, a roughly 35 m deep, 100??50 km embayment on the northeastern shelf of the United States. Using a variant of the Blumberg-Mellor (1987) model, it was found that a continuous 18 month run was only possible if the velocity field was Shapiro filtered to remove two grid length energy that developed along the open boundary due to mismatch in locally generated and climatologically forced water properties. The seasonal development of temperature and salinity stratification was well-represented by the model once ??-coordinate errors were reduced by subtracting domain averaged vertical profiles of temperature, salinity and density before horizontal differencing was performed. Comparison of modeled and observed subtidal currents at fixed locations revealed that the model performance varies strongly with season and distance from the open boundaries. The model performs best during unstratified conditions, and in the interior of the bay. The model performs poorest during stratified conditions and in the regions where the bay is driven predominantly by remote fluctuations from the Gulf of Maine.

Impact of Martial Arts (Judo, Karate, and Kung Fu) on Bone Mineral Density Gains in Adolescents of Both Genders: 9-Month Follow-Up.

PubMed

Ito, Igor H; Kemper, Han C G; Agostinete, Ricardo R; Lynch, Kyle R; Christofaro, Diego G D; Ronque, Enio R; Fernandes, Rômulo A

2017-11-01

To compare bone mineral density (BMD) gains in adolescents of both genders stratified according to different martial art styles in a 9-month follow-up study. The longitudinal study consisted of 29 adolescents of both genders and age between 11 and 17 years stratified into a control group (not engaged in any sport) and 50 fighters (kung fu/karate, n = 29; judo, n = 21). All 79 subjects underwent anthropometric measures (weight, height, leg length, and height set) and dual-energy X-ray absorptiometry (BMD, in g/cm 2 ) at 2 moments, baseline and 9 months later. Maturity offset (age at peak height velocity), lean soft tissue, chronological age, and resistance training were treated as covariates. Male judoists presented higher gains in BMD-spine [0.098 g/cm 2 (95% confidence interval, 0.068-0.128)] than control group [0.040 g/cm 2 (95% confidence interval, 0.011-0.069)] (post hoc test with P = .030). There was no effect of martial art on BMD gains among girls. Independently of gender, in all multivariate models, lean soft tissue constituted the most relevant covariate. Judo practice in adolescents affected the bone accrual significantly after 9-month follow-up compared with controls, mainly in boys.

Theory of hyperbolic stratified nanostructures for surface-enhanced Raman scattering

NASA Astrophysics Data System (ADS)

Wong, Herman M. K.; Dezfouli, Mohsen Kamandar; Axelrod, Simon; Hughes, Stephen; Helmy, Amr S.

2017-11-01

We theoretically investigate the enhancement of surface enhanced Raman spectroscopy (SERS) using hyperbolic stratified nanostructures and compare to metal nanoresonators. The photon Green function of each nanostructure within its environment is first obtained from a semianalytical modal theory, which is used in a quantum optics formalism of the molecule-nanostructure interaction to model the SERS spectrum. An intuitive methodology is presented for calculating the single-molecule enhancement factor (SMEF), which is also able to predict known experimental SERS enhancement factors of a gold nanodimer. We elucidate the important figures-of-merit of the enhancement and explore these for different designs. We find that the use of hyperbolic stratified materials can enhance the photonic local density of states (LDOS) by close to two times in comparison to pure metal nanostructures, when both designed to work at the same operating wavelengths. However, the increased LDOS is accompanied by higher electric field concentration within the lossy hyperbolic material, which leads to increased quenching that serves to reduce the overall detected SERS enhancement in the far field. For nanoresonators with resonant localized surface plasmon wavelengths in the near-infrared, the SMEF for the hyperbolic stratified nanostructure is approximately one order of magnitude lower than the pure metal counterpart. Conversely, we show that by detecting the Raman signal using a near-field probe, hyperbolic materials can provide an improvement in SERS enhancement compared to using pure metal nanostructures when the probe is sufficiently close (<50 nm ) to the Raman active molecule at the plasmonic hotspot.

Evolution of Starburst Galaxies in the Illustris Simulation

NASA Astrophysics Data System (ADS)

Wilkinson, C. L.; Pimbblet, K. A.; Stott, J. P.; Few, C. G.; Gibson, B. K.

2018-06-01

There is a consensus in the literature that starburst galaxies are triggered by interaction events. However, it remains an open question as to what extent both merging and non-merging interactions have in triggering starbursts? In this study, we make use of the Illustris simulation to test how different triggering mechanisms can effect starburst events. We examine star formation rate, colour and environment of starburst galaxies to determine if this could be why we witness a bimodality in post-starburst populations within observational studies. Further, we briefly test the extent of quenching due to AGN feedback. From Illustris, we select 196 starburst galaxies at z = 0.15 and split them into post-merger and pre-merger/harassment driven starburst samples. We find that 55% of this sample not undergone a merger in the past 2 Gyr. Both of our samples are located in low-density environments within the filament regions of the cosmic web, however we find that pre-merger/harassment driven starburst are in higher density environments than post-merger driven starbursts. We also find that pre-merger/harassment starbursts are redder than post-merger starbursts, this could be driven by environmental effects. Both however, produce nuclear starbursts of comparable strengths.

Modification of ocean-estuary salt fluxes by density-driven advection of a headland eddy

NASA Astrophysics Data System (ADS)

Fram, J. P.; Stacey, M. T.

2005-05-01

Scalar exchange between San Francisco Bay and the coastal ocean is examined using shipboard observations made across the Golden Gate Channel. Ocean-estuary exchange is often described as a combination of two independent types of mechanisms: density-driven exchange such as gravitational circulation and tidal asymmetries such as tidal trapping. In this study we found that exchange is also governed by an interaction between these mechanisms. Tidally trapped eddies created in shallow shoals are mixed into the main channel earlier in the tidal cycle during the rainy season because the eddies are pushed seaward by gravitational circulation. This interaction increases the tidally averaged dispersive salt flux into the bay. The study consists of experiments during each of three 'seasons': winter/spring runoff (March 2002), summer upwelling (July 2003), and fall relaxation (October 2002). Within each experiment, transects across the channel were repeated approximately every 12 minutes for 25 hours during both spring tide and the following neap tide. Velocity was measured from a boat-mounted ADCP. Scalar concentrations were measured from a tow-yoed SeaSciences Acrobat. Salinity exchange over each spring-neap cycle is quantified with harmonic analysis. Harmonic results are decomposed into flux mechanisms using temporal and spatial correlations. The temporal correlation of cross-sectional averaged salinity and velocity (tidal pumping flux) is the largest part of the dispersive flux of salinity into the bay. From the tidal pumping portion of the dispersive flux, it is shown that there is less exchange than was found in earlier studies. Furthermore, tidal pumping flux scales strongly with flow due to density-driven movement of tidally trapped eddies and density-driven increases in ebb-flood frictional phasing. Complex bathymetry makes salinity exchange scale differently with flow than would be expected from simple tidal pumping and gravitational circulation models.

Tidal-Induced Ocean Dynamics as Cause of Enceladus' Tiger Stripe Pattern

NASA Astrophysics Data System (ADS)

Vermeersen, B. L.; Maas, L. R.; van Oers, S.; Rabitti, A.; Jara-Orue, H.

2013-12-01

One of the most peculiar features on Saturn moon Enceladus is its so-called tiger stripe pattern at the geologically active South Polar Terrain (SPT), as first observed in detail by the Cassini spacecraft early 2005. It is generally assumed that the four almost parallel surface lines that constitute this pattern are faults in the icy surface overlying a confined salty water reservoir. Indeed, later Cassini observations have shown that salty water jets originate from the tiger stripes [e.g., Hansen et al., Science, 311, 1422-1425, 2006; Postberg et al., Nature, 474, 620-622, 2011]. The periodic activity of the tiger stripe faults shows a strong correlation with tidal forcing. Jets emanating from specific fault lines seem to be triggered at those places of the faults where tidal-induced stresses are largest immediately following closest orbital approach with Saturn [e.g., Hurford et al., Nature, 447, 292-294, 2007]. Thus jet activity seems to be directly induced by tidal forcing. However, this does not explain the characteristic regular pattern of the stripes themselves. Here we explore the possibility that this pattern is formed and maintained by induced, tidally and rotationally driven, fluid motions in the ocean underneath the icy surface of the tiger-stripe region. The remarkable spatial regularity of Enceladus' SPT fault lines is reminiscent of that observed at the surface of confined density-stratified fluids by the action of induced internal gravity waves. Theoretical analysis, numerical simulations and laboratory water tank experiments all indicate that wave attractors - particular limit orbits to which waves are focused in a fluid basin - naturally emerge in gravitationally (radial salt concentration or temperature differences) or rotationally stratified confined fluids as a function of forcing periodicity and fluid basin geometry [Maas et al., Nature, 338, 557-561, 1997]. We have found that ocean dynamical wave attractors induced by tidal-effective forcing of Enceladus' SPT salty water basin can reproduce the general characteristics of the observed tiger stripe pattern and even offer the possibility of constraining the 3D-form of the salty water basin underlying Enceladus' SPT. Vertical cross section of one of the water tank experiments. The tank is uniformly stratified with salty water and harmonically shaken. Wave attractors impinge at the surface of the tank at A, B and C, which are places where an overlying plate experiences enhanced stress levels. Distances A-B and B-C are not the same due to a sloping floor of the fluid tank. The length of the tank is about 1.5 m. Numbers at the bottom indicate mm.

Density and distribution of nitrifying guilds in rapid sand filters for drinking water production: Dominance of Nitrospira spp.

PubMed

Tatari, Karolina; Musovic, Sanin; Gülay, Arda; Dechesne, Arnaud; Albrechtsen, Hans-Jørgen; Smets, Barth F

2017-12-15

We investigated the density and distribution of total bacteria, canonical Ammonia Oxidizing Bacteria (AOB) (Nitrosomonas plus Nitrosospira), Ammonia Oxidizing Archaea (AOA), as well as Nitrobacter and Nitrospira in rapid sand filters used for groundwater treatment. To investigate the spatial distribution of these guilds, filter material was sampled at four drinking water treatment plants (DWTPs) in parallel filters of the pre- and after-filtration stages at different locations and depths. The target guilds were quantified by qPCR targeting 16S rRNA and amoA genes. Total bacterial densities (ignoring 16S rRNA gene copy number variation) were high and ranged from 10 9 to 10 10 per gram (10 15 to 10 16 per m 3 ) of filter material. All examined guilds, except AOA, were stratified at only one of the four DWTPs. Densities varied spatially within filter (intra-filter variation) at two of the DWTPs and in parallel filters (inter-filter variation) at one of the DWTPs. Variation analysis revealed random sampling as the most efficient strategy to yield accurate mean density estimates, with collection of at least 7 samples suggested to obtain an acceptable (below half order of magnitude) density precision. Nitrospira was consistently the most dominant guild (5-10% of total community), and was generally up to 4 orders of magnitude more abundant than Nitrobacter and up to 2 orders of magnitude more abundant than canonical AOBs. These results, supplemented with further analysis of the previously reported diversity of Nitrospira in the studied DWTPs based on 16S rRNA and nxrB gene phylogeny (Gülay et al., 2016; Palomo et al., 2016), indicate that the high Nitrospira abundance is due to their comammox (complete ammonia oxidation) physiology. AOA densities were lower than AOB densities, except in the highly stratified filters, where they were of similar abundance. In conclusion, rapid sand filters are microbially dense, with varying degrees of spatial heterogeneity, which requires replicate sampling for a sufficiently precise determination of total microbial community and specific population densities. A consistently high Nitrospira to bacterial and archaeal AOB density ratio suggests that non-canonical pathways for nitrification may dominate the examined RSFs. Copyright © 2017 Elsevier Ltd. All rights reserved.

Freshwater discharges drive high levels of methylmercury in Arctic marine biota.

PubMed

Schartup, Amina T; Balcom, Prentiss H; Soerensen, Anne L; Gosnell, Kathleen J; Calder, Ryan S D; Mason, Robert P; Sunderland, Elsie M

2015-09-22

Elevated levels of neurotoxic methylmercury in Arctic food-webs pose health risks for indigenous populations that consume large quantities of marine mammals and fish. Estuaries provide critical hunting and fishing territory for these populations, and, until recently, benthic sediment was thought to be the main methylmercury source for coastal fish. New hydroelectric developments are being proposed in many northern ecosystems, and the ecological impacts of this industry relative to accelerating climate changes are poorly characterized. Here we evaluate the competing impacts of climate-driven changes in northern ecosystems and reservoir flooding on methylmercury production and bioaccumulation through a case study of a stratified sub-Arctic estuarine fjord in Labrador, Canada. Methylmercury bioaccumulation in zooplankton is higher than in midlatitude ecosystems. Direct measurements and modeling show that currently the largest methylmercury source is production in oxic surface seawater. Water-column methylation is highest in stratified surface waters near the river mouth because of the stimulating effects of terrestrial organic matter on methylating microbes. We attribute enhanced biomagnification in plankton to a thin layer of marine snow widely observed in stratified systems that concentrates microbial methylation and multiple trophic levels of zooplankton in a vertically restricted zone. Large freshwater inputs and the extensive Arctic Ocean continental shelf mean these processes are likely widespread and will be enhanced by future increases in water-column stratification, exacerbating high biological methylmercury concentrations. Soil flooding experiments indicate that near-term changes expected from reservoir creation will increase methylmercury inputs to the estuary by 25-200%, overwhelming climate-driven changes over the next decade.

Magnetic fields driven by tidal mixing in radiative stars

NASA Astrophysics Data System (ADS)

Vidal, Jérémie; Cébron, David; Schaeffer, Nathanaël; Hollerbach, Rainer

2018-04-01

Stellar magnetism plays an important role in stellar evolution theory. Approximatively 10 per cent of observed main sequence (MS) and pre-main-sequence (PMS) radiative stars exhibit surface magnetic fields above the detection limit, raising the question of their origin. These stars host outer radiative envelopes, which are stably stratified. Therefore, they are assumed to be motionless in standard models of stellar structure and evolution. We focus on rapidly rotating, radiative stars which may be prone to the tidal instability, due to an orbital companion. Using direct numerical simulations in a sphere, we study the interplay between a stable stratification and the tidal instability, and assess its dynamo capability. We show that the tidal instability is triggered regardless of the strength of the stratification (Brunt-Väisälä frequency). Furthermore, the tidal instability can lead to both mixing and self-induced magnetic fields in stably stratified layers (provided that the Brunt-Väisälä frequency does not exceed the stellar spin rate in the simulations too much). The application to stars suggests that the resulting magnetic fields could be observable at the stellar surfaces. Indeed, we expect magnetic field strengths up to several Gauss. Consequently, tidally driven dynamos should be considered as a (complementary) dynamo mechanism, possibly operating in radiative MS and PMS stars hosting orbital companions. In particular, tidally driven dynamos may explain the observed magnetism of tidally deformed and rapidly rotating Vega-like stars.

Stochastic transport models for mixing in variable-density turbulence

NASA Astrophysics Data System (ADS)

Bakosi, J.; Ristorcelli, J. R.

2011-11-01

In variable-density (VD) turbulent mixing, where very-different- density materials coexist, the density fluctuations can be an order of magnitude larger than their mean. Density fluctuations are non-negligible in the inertia terms of the Navier-Stokes equation which has both quadratic and cubic nonlinearities. Very different mixing rates of different materials give rise to large differential accelerations and some fundamentally new physics that is not seen in constant-density turbulence. In VD flows material mixing is active in a sense far stronger than that applied in the Boussinesq approximation of buoyantly-driven flows: the mass fraction fluctuations are coupled to each other and to the fluid momentum. Statistical modeling of VD mixing requires accounting for basic constraints that are not important in the small-density-fluctuation passive-scalar-mixing approximation: the unit-sum of mass fractions, bounded sample space, and the highly skewed nature of the probability densities become essential. We derive a transport equation for the joint probability of mass fractions, equivalent to a system of stochastic differential equations, that is consistent with VD mixing in multi-component turbulence and consistently reduces to passive scalar mixing in constant-density flows.

Interactions of task and subject variables among continuous performance tests.

PubMed

Denney, Colin B; Rapport, Mark D; Chung, Kyong-Mee

2005-04-01

Contemporary models of working memory suggest that target paradigm (TP) and target density (TD) should interact as influences on error rates derived from continuous performance tests (CPTs). The present study evaluated this hypothesis empirically in a typically developing, ethnically diverse sample of children. The extent to which scores based on different combinations of these task parameters showed different patterns of relationship to age, intelligence, and gender was also assessed. Four continuous performance tests were derived by combining two target paradigms (AX and repeated letter target stimuli) with two levels of target density (8.3% and 33%). Variations in mean omission (OE) and commission (CE) error rates were examined within and across combinations of TP and TD. In addition, a nested series of structural equation models was utilized to examine patterns of relationship among error rates, age, intelligence, and gender. Target paradigm and target density interacted as influences on error rates. Increasing density resulted in higher OE and CE rates for the AX paradigm. In contrast, the high density condition yielded a decline in OE rates accompanied by a small increase in CEs using the repeated letter CPT. Target paradigms were also distinguishable on the basis of age when using OEs as the performance measure, whereas combinations of age and intelligence distinguished between density levels but not target paradigms using CEs as the dependent measure. Different combinations of target paradigm and target density appear to yield scores that are conceptually and psychometrically distinguishable. Consequently, developmentally appropriate interpretation of error rates across tasks may require (a) careful analysis of working memory and attentional resources required for successful performance, and (b) normative data bases that are differently stratified with respect to combinations of age and intelligence.

Kinetic instability of electrostatic ion cyclotron waves in inter-penetrating plasmas

NASA Astrophysics Data System (ADS)

Bashir, M. F.; Ilie, R.; Murtaza, G.

2018-05-01

The Electrostatic Ion Cyclotron (EIC) instability that includes the effect of wave-particle interaction is studied owing to the free energy source through the flowing velocity of the inter-penetrating plasmas. It is shown that the origin of this current-less instability is different from the classical current driven EIC instability. The threshold conditions applicable to a wide range of plasma parameters and the estimate of the growth rate are determined as a function of the normalized flowing velocity ( u0/vt f e ), the temperature ( Tf/Ts ) and the density ratios ( nf 0/ns 0 ) of flowing component to static one. The EIC instability is driven by either flowing electrons or flowing ions, depending upon the different Doppler shifted frequency domains. It is found that the growth rate for electron-driven instability is higher than the ion-driven one. However, in both cases, the denser (hotter) is the flowing plasma, the lesser (greater) is the growth rate. The possible applications related to the terrestrial solar plasma environment are also discussed.

m-BIRCH: an online clustering approach for computer vision applications

NASA Astrophysics Data System (ADS)

Madan, Siddharth K.; Dana, Kristin J.

2015-03-01

We adapt a classic online clustering algorithm called Balanced Iterative Reducing and Clustering using Hierarchies (BIRCH), to incrementally cluster large datasets of features commonly used in multimedia and computer vision. We call the adapted version modified-BIRCH (m-BIRCH). The algorithm uses only a fraction of the dataset memory to perform clustering, and updates the clustering decisions when new data comes in. Modifications made in m-BIRCH enable data driven parameter selection and effectively handle varying density regions in the feature space. Data driven parameter selection automatically controls the level of coarseness of the data summarization. Effective handling of varying density regions is necessary to well represent the different density regions in data summarization. We use m-BIRCH to cluster 840K color SIFT descriptors, and 60K outlier corrupted grayscale patches. We use the algorithm to cluster datasets consisting of challenging non-convex clustering patterns. Our implementation of the algorithm provides an useful clustering tool and is made publicly available.

Density-driven vertical transport of saltwater through the freshwater lens on the island of Baltrum (Germany) following the 1962 storm flood

NASA Astrophysics Data System (ADS)

Post, Vincent E. A.; Houben, Georg J.

2017-08-01

Due to the growing vulnerability of low-lying coastal zones to flooding by seawater, there is a current need for studies of the impact of such inundations on fresh groundwater resources. The knowledge from the literature is biased towards tropical atoll environments, and only few studies specifically investigated the effect of density-driven downward flow, even though its importance is widely acknowledged. The present study is based on previously unpublished hydrochemical data collected on the island of Baltrum following a devastating storm in 1962, which uniquely show the impact of seawater inundation on a freshwater lens in a siliciclastic aquifer. The field data show that about 3 kg of Cl per m2 of inundated land area, or 18 cm of seawater, infiltrated, and that elevated salinities persisted at the measurement depths of 4 and 6 m for at least 4 years, and at least for 6 years at greater depths. Numerical models support the assertion that the shape of the measured salinographs, i.e. an initial sharp rise in the salt concentration with time, followed by a continually-slowing decrease, must be attributed to density-driven salt fingering. Models that did not consider density effects fail to simulate the observed patterns. Transient recharge, model dimension and lateral flow modify the details of the simulation results, but in all models density-driven vertical flow dominates the overall system behaviour. The diminishing importance of density-driven flow at greater depths, however, in combination with slow recharge-driven flow rates prolongs flushing times, and enhances the risk of brackish-water up-coning when pumping is resumed too soon.

Modulation of basal cell fate during productive and transforming HPV-16 infection is mediated by progressive E6-driven depletion of Notch.

PubMed

Kranjec, Christian; Holleywood, Christina; Libert, Diane; Griffin, Heather; Mahmood, Radma; Isaacson, Erin; Doorbar, John

2017-08-01

In stratified epithelia such as the epidermis, homeostasis is maintained by the proliferation of cells in the lower epithelial layers and the concomitant loss of differentiated cells from the epithelial surface. These differentiating keratinocytes progressively stratify and form a self-regenerating multi-layered barrier that protects the underlying dermis. In such tissue, the continual loss and replacement of differentiated cells also limits the accumulation of oncogenic mutations within the tissue. Inactivating mutations in key driver genes, such as TP53 and NOTCH1, reduce the proportion of differentiating cells allowing for the long-term persistence of expanding mutant clones in the tissue. Here we show that through the expression of E6, HPV-16 prevents the early fate commitment of human keratinocytes towards differentiation and confers a strong growth advantage to human keratinocytes. When E6 is expressed either alone or with E7, it promotes keratinocyte proliferation at high cell densities, through the combined inactivation of p53 and Notch1. In organotypic raft culture, the activity of E6 is restricted to the basal layer of the epithelium and is enhanced during the progression from productive to abortive or transforming HPV-16 infection. Consistent with this, the expression of p53 and cleaved Notch1 becomes progressively more disrupted, and is associated with increased basal cell density and reduced commitment to differentiation. The expression of cleaved Notch1 is similarly disrupted also in HPV-16-positive cervical lesions, depending on neoplastic grade. When taken together, these data depict an important role of high-risk E6 in promoting the persistence of infected keratinocytes in the basal and parabasal layers through the inactivation of gene products that are commonly mutated in non-HPV-associated neoplastic squamous epithelia. © 2017 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland. © 2017 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Acceleration of on-axis and ring-shaped electron beams in wakefields driven by Laguerre-Gaussian pulses

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Guo-Bo; Key Laboratory for Laser Plasmas; Chen, Min, E-mail: minchen@sjtu.edu.cn, E-mail: yanyunma@126.com

2016-03-14

The acceleration of electron beams with multiple transverse structures in wakefields driven by Laguerre-Gaussian pulses has been studied through three-dimensional (3D) particle-in-cell simulations. Under different laser-plasma conditions, the wakefield shows different transverse structures. In general cases, the wakefield shows a donut-like structure and it accelerates the ring-shaped hollow electron beam. When a lower plasma density or a smaller laser spot size is used, besides the donut-like wakefield, a central bell-like wakefield can also be excited. The wake sets in the center of the donut-like wake. In this case, both a central on-axis electron beam and a ring-shaped electron beam aremore » simultaneously accelerated. Further, reducing the plasma density or laser spot size leads to an on-axis electron beam acceleration only. The research is beneficial for some potential applications requiring special pulse beam structures, such as positron acceleration and collimation.« less

Vertical migration and nighttime distribution of adult bloaters in Lake Michigan

USGS Publications Warehouse

TeWinkel, Leslie M.; Fleischer, Guy W.

1999-01-01

The vertical migration and nighttime vertical distribution of adult bloaters Coregonus hoyi were investigated during late summer in Lake Michigan using acoustics simultaneously with either midwater or bottom trawling. Bloaters remained on or near bottom during the day. At night, bloaters were distributed throughout 30-65 m of water, depending on bottom depth. Shallowest depths of migration were not related to water temperature or incident light. Maximum distances of migration increased with increasing bottom depth. Nighttime midwater densities ranged from 0.00 to 6.61 fish/1,000 mA? and decreased with increasing bottom depth. Comparisons of length distributions showed that migrating and nonmigrating bloaters did not differ in size. However, at most sites, daytime bottom catches collected a greater proportion of larger individuals compared with nighttime midwater or bottom catches. Mean target strengths by 5-m strata indicated that migrating bloaters did not stratify by size in the water column at night. Overall, patterns in frequency of empty stomachs and mean digestive state of prey indicated that a portion of the bloater population fed in the water column at night. Bloater diet composition indicated both midwater feeding and bottom feeding. In sum, although a portion of the bloater population fed in the water column at night, bloaters were not limited to feeding at this time. This research confirmed that bloaters are opportunistic feeders and did not fully support the previously proposed hypothesis that bloater vertical migration is driven by the vertically migrating macroinvertebrate the opossom shrimp Mysis relicta.

Estimate of size distribution of charged MSPs measured in situ in winter during the WADIS-2 sounding rocket campaign

NASA Astrophysics Data System (ADS)

Asmus, Heiner; Staszak, Tristan; Strelnikov, Boris; Lübken, Franz-Josef; Friedrich, Martin; Rapp, Markus

2017-08-01

We present results of in situ measurements of mesosphere-lower thermosphere dusty-plasma densities including electrons, positive ions and charged aerosols conducted during the WADIS-2 sounding rocket campaign. The neutral air density was also measured, allowing for robust derivation of turbulence energy dissipation rates. A unique feature of these measurements is that they were done in a true common volume and with high spatial resolution. This allows for a reliable derivation of mean sizes and a size distribution function for the charged meteor smoke particles (MSPs). The mean particle radius derived from Schmidt numbers obtained from electron density fluctuations was ˜ 0.56 nm. We assumed a lognormal size distribution of the charged meteor smoke particles and derived the distribution width of 1.66 based on in situ-measured densities of different plasma constituents. We found that layers of enhanced meteor smoke particles' density measured by the particle detector coincide with enhanced Schmidt numbers obtained from the electron and neutral density fluctuations. Thus, we found that large particles with sizes > 1 nm were stratified in layers of ˜ 1 km thickness and lying some kilometers apart from each other.

The Progenitor Dependence of Core-collapse Supernovae from Three-dimensional Simulations with Progenitor Models of 12–40 M ⊙

NASA Astrophysics Data System (ADS)

Ott, Christian D.; Roberts, Luke F.; da Silva Schneider, André; Fedrow, Joseph M.; Haas, Roland; Schnetter, Erik

2018-03-01

We present a first study of the progenitor star dependence of the three-dimensional (3D) neutrino mechanism of core-collapse supernovae. We employ full 3D general-relativistic multi-group neutrino radiation-hydrodynamics and simulate the postbounce evolutions of progenitors with zero-age main sequence masses of 12, 15, 20, 27, and 40 M ⊙. All progenitors, with the exception of the 12 M ⊙ star, experience shock runaway by the end of their simulations. In most cases, a strongly asymmetric explosion will result. We find three qualitatively distinct evolutions that suggest a complex dependence of explosion dynamics on progenitor density structure, neutrino heating, and 3D flow. (1) Progenitors with massive cores, shallow density profiles, and high post-core-bounce accretion rates experience very strong neutrino heating and neutrino-driven turbulent convection, leading to early shock runaway. Accretion continues at a high rate, likely leading to black hole formation. (2) Intermediate progenitors experience neutrino-driven, turbulence-aided explosions triggered by the arrival of density discontinuities at the shock. These occur typically at the silicon/silicon–oxygen shell boundary. (3) Progenitors with small cores and density profiles without strong discontinuities experience shock recession and develop the 3D standing-accretion shock instability (SASI). Shock runaway ensues late, once declining accretion rate, SASI, and neutrino-driven convection create favorable conditions. These differences in explosion times and dynamics result in a non-monotonic relationship between progenitor and compact remnant mass.

Subsurface flows in the seasonally stratified central North Sea : analysis of drifter tracks through observations and modelling

NASA Astrophysics Data System (ADS)

Chambers, C.; McCloghrie, P.; Fernand, L.; Brown, J.; Young, E. F.

2003-04-01

Holey-sock drifters have been tracked by ARGOS satellite in the Central North Sea during summer-stratified conditions of 1996, 1997, 1999, 2001 and 2002. Drogued at depths of 20-30m, they aim to capture the baroclinic jets set up by isolated cold pool bottom fronts. These cold pools of relict winter water remain through the summer in areas of low tidal energy and are effectively sealed off from overlying waters by a strong thermocline. Observational and modelling studies have identified such dynamics in the basins both north of the Dogger Bank - Fladen Grounds - and south - Oyster Grounds. The drifter tracks used in this study were interpolated and tidally filtered to produce regular time interval drifter positions. By correlation with wind data from the UK Meteorological Office Unified Model output, the locally wind-driven and baroclinic components of the drifters' flow were determined. Following assessments of (1) individual drifter tracks and (2) spatial/temporal segmentation of the collective drifter tracks, a regional and interannual understanding of the area has been built up. Additional observational data (including that gathered with high resolution towed undulating CTD's on a Scanfish) have been used to support and quantify the flows, as has a 3-D density-resolving model based on the Princeton Ocean Model (POM). The drifters have been simulated using a particle-tracking model run on POM's flow field output, simulating the paths of drifters at depth. Through running the two together in various modes, it has been possible to account for certain parts of the drifters' tracks. These results contribute to a previously coarser understanding of North Sea circulation and show the importance of seasonal structure there. They demonstrate that fast baroclinic jets have the potential to transport biological and contaminant matter (e.g., fish larvae/eggs; and nutrients/heavy metals) in different and more organised flow fields than those previously recognised. This understanding is essential to the monitoring and management of such a semi-enclosed and intensively used area as the North Sea.

Interactions between gravity waves and cold air outflows in a stably stratified uniform flow

NASA Technical Reports Server (NTRS)

Lin, Yuh-Lang; Wang, Ting-An; Weglarz, Ronald P.

1993-01-01

Interactions between gravity waves and cold air outflows in a stably stratified uniform flow forced by various combinations of prescribed heat sinks and sources are studied using a hydrostatic two-dimensional nonlinear numerical model. The formation time for the development of a stagnation point or reversed flow at the surface is not always directly proportional to the Froude number when wave reflections exist from upper levels. A density current is able to form by the wave-otuflow interaction, even though the Froude number is greater than a critical value. This is the result of the wave-outflow interaction shifting the flow response to a different location in the characteristic parameter space. A density current is able to form or be destroyed due to the wave-outflow interaction between a traveling gravity wave and cold air outflow. This is proved by performing experiments with a steady-state heat sink and an additional transient heat source. In a quiescent fluid, a region of cold air, convergence, and upward motion is formed after the collision between two outflows produced by two prescribed heat sinks. After the collision, the individual cold air outflows lose their own identity and merge into a single, stationary, cold air outflow region. Gravity waves tend to suppress this new stationary cold air outflow after the collision. The region of upward motion associated with the collision is confined to a very shallow layer. In a moving airstream, a density current produced by a heat sink may be suppressed or enhanced nonlinearly by an adjacent heat sink due to the wave-outflow interaction.

Mean-field theory of differential rotation in density stratified turbulent convection

NASA Astrophysics Data System (ADS)

Rogachevskii, I.

2018-04-01

A mean-field theory of differential rotation in a density stratified turbulent convection has been developed. This theory is based on the combined effects of the turbulent heat flux and anisotropy of turbulent convection on the Reynolds stress. A coupled system of dynamical budget equations consisting in the equations for the Reynolds stress, the entropy fluctuations and the turbulent heat flux has been solved. To close the system of these equations, the spectral approach, which is valid for large Reynolds and Péclet numbers, has been applied. The adopted model of the background turbulent convection takes into account an increase of the turbulence anisotropy and a decrease of the turbulent correlation time with the rotation rate. This theory yields the radial profile of the differential rotation which is in agreement with that for the solar differential rotation.

Effects of ICRF power on SOL density profiles and LH coupling during simultaneous LH and ICRF operation on Alcator C-Mod

NASA Astrophysics Data System (ADS)

Lau, C.; Lin, Y.; Wallace, G.; Wukitch, S. J.; Hanson, G. R.; Labombard, B.; Ochoukov, R.; Shiraiwa, S.; Terry, J.

2013-09-01

A dedicated experiment during simultaneous lower hybrid (LH) and ion cyclotron range-of-frequencies (ICRF) operations is carried out to evaluate and understand the effects of ICRF power on the scrape-off-layer (SOL) density profiles and on the resultant LH coupling for a wide range of plasma parameters on Alcator C-Mod. Operation of the LH launcher with the adjacent ICRF antenna significantly degrades LH coupling while operation with the ICRF antenna that is not magnetically connected to the LH launcher minimally affects LH coupling. An X-mode reflectometer system at three poloidal locations adjacent to the LH launcher and a visible video camera imaging the LH launcher are used to measure local SOL density profile and emissivity modifications with the application of LH and LH + ICRF power. These measurements confirm that the density in front of the LH launcher depends strongly on the magnetic field line mapping of the active ICRF antenna. Reflectometer measurements also observe both ICRF-driven and LH-driven poloidal density profile asymmetries, especially a strong density depletion at certain poloidal locations in front of the LH launcher during operation with a magnetically connected ICRF antenna. The results indicate that understanding both LH-driven flows and ICRF sheath driven flows may be necessary to understand the observed density profile modifications and LH coupling results during simultaneous LH + ICRF operation.

Global simulations of protoplanetary disks with net magnetic flux. I. Non-ideal MHD case

NASA Astrophysics Data System (ADS)

Béthune, William; Lesur, Geoffroy; Ferreira, Jonathan

2017-04-01

Context. The planet-forming region of protoplanetary disks is cold, dense, and therefore weakly ionized. For this reason, magnetohydrodynamic (MHD) turbulence is thought to be mostly absent, and another mechanism has to be found to explain gas accretion. It has been proposed that magnetized winds, launched from the ionized disk surface, could drive accretion in the presence of a large-scale magnetic field. Aims: The efficiency and the impact of these surface winds on the disk structure is still highly uncertain. We present the first global simulations of a weakly ionized disk that exhibits large-scale magnetized winds. We also study the impact of self-organization, which was previously demonstrated only in non-stratified models. Methods: We perform numerical simulations of stratified disks with the PLUTO code. We compute the ionization fraction dynamically, and account for all three non-ideal MHD effects: ohmic and ambipolar diffusions, and the Hall drift. Simplified heating and cooling due to non-thermal radiation is also taken into account in the disk atmosphere. Results: We find that disks can be accreting or not, depending on the configuration of the large-scale magnetic field. Magnetothermal winds, driven both by magnetic acceleration and heating of the atmosphere, are obtained in the accreting case. In some cases, these winds are asymmetric, ejecting predominantly on one side of the disk. The wind mass loss rate depends primarily on the average ratio of magnetic to thermal pressure in the disk midplane. The non-accreting case is characterized by a meridional circulation, with accretion layers at the disk surface and decretion in the midplane. Finally, we observe self-organization, resulting in axisymmetric rings of density and associated pressure "bumps". The underlying mechanism and its impact on observable structures are discussed.

Stability analysis of wall driven nanofluid flow through a tube

NASA Astrophysics Data System (ADS)

Hossain, M. Mainul; Khan, M. A. H.

2017-06-01

Wall driven incompressible viscous fluid flow with nanoparticles through a tube is considered where two different nanofluids (Cu-water, SiO2-water) are used separately. Flow becomes gradually unstable due to movement of wall and existence of nanoparticles. However, Reynolds number, volume fraction and density ratio are responsible for flow instability. The mathematical model of the problem is constructed and solved by means of series solution method. Special type Hermite-Padé approximation method is used to improve the series solution. The critical point for Reynolds number, volume fraction and density ratio are determined and described using approximation technique and bifurcation diagram for both nanofluids. Moreover, Interaction between these three numbers and their effect on velocity profile are discussed. To indicate the nanofluid which is more effective for flow stability is our major concerned.

X-ray source characterization of aluminum X-pinch plasmas driven by the 0. 5 TW LION accelerator

DOE Office of Scientific and Technical Information (OSTI.GOV)

Qi, N.; Hammer, D.A.; Kalantar, D.H.

1989-12-01

Recent experiments at Cornell have been performed to investigate X-pinch plasmas as intense x-ray sources which might be used to pump resonant photoexcitation lasers. Crossed Al wires have been driven by up to 600 kA current for 40 ns. High density bright spots are observed at the crossing point(s). Various diagnostics were used to characterize the X-pinch plasmas as a function of initial mass loading for several specific wire configurations. The optimum mass loading for different ionization stages of Al, and the total x-ray energy yields, which are on the order of hundreds of Joules, were examined. Estimates of plasmamore » density, {similar to}10{sup 20} cm{sup {minus}3}, and temperature, about 400 eV, were obtained.« less

A 1D ion species model for an RF driven negative ion source

NASA Astrophysics Data System (ADS)

Turner, I.; Holmes, A. J. T.

2017-08-01

A one-dimensional model for an RF driven negative ion source has been developed based on an inductive discharge. The RF source differs from traditional filament and arc ion sources because there are no primary electrons present, and is simply composed of an antenna region (driver) and a main plasma discharge region. However the model does still make use of the classical plasma transport equations for particle energy and flow, which have previously worked well for modelling DC driven sources. The model has been developed primarily to model the Small Negative Ion Facility (SNIF) ion source at CCFE, but may be easily adapted to model other RF sources. Currently the model considers the hydrogen ion species, and provides a detailed description of the plasma parameters along the source axis, i.e. plasma temperature, density and potential, as well as current densities and species fluxes. The inputs to the model are currently the RF power, the magnetic filter field and the source gas pressure. Results from the model are presented and where possible compared to existing experimental data from SNIF, with varying RF power, source pressure.

Evidence of a New Instability in Gyrokinetic Simulations of LAPD Plasmas

NASA Astrophysics Data System (ADS)

Terry, P. W.; Pueschel, M. J.; Rossi, G.; Jenko, F.; Told, D.; Carter, T. A.

2015-11-01

Recent experiments at the LArge Plasma Device (LAPD) have focused on structure formation driven by density and temperature gradients. A central difference relative to typical, tokamak-like plasmas stems from the linear geometry and absence of background magnetic shear. At sufficiently high β, strong excitation of parallel (compressional) magnetic fluctuations was observed. Here, linear and nonlinear simulations with the Gene code are used to demonstrate that these findings can be explained through the linear excitation of a Gradient-driven Drift Coupling mode (GDC). This recently-discovered instability, unlike other drift waves, relies on the grad-B drift due to parallel magnetic fluctuations in lieu of a parallel electron response, and can be driven by density or temperature gradients. The linear properties of the GDC for LAPD parameters are studied in detail, and the corresponding turbulence is investigated. It is found that, despite the very large collisionality in the experiment, many properties are recovered fairly well in the simulations. In addition to confirming the existence of the GDC, this opens up interesting questions regarding GDC activity in astrophysical and space plasmas. Supported by USDOE.

What Does a Submillimeter Galaxy Selection Actually Select? The Dependence of Submillimeter Flux Density on Star Formation Rate and Dust Mass

NASA Astrophysics Data System (ADS)

Hayward, Christopher C.; Kereš, Dušan; Jonsson, Patrik; Narayanan, Desika; Cox, T. J.; Hernquist, Lars

2011-12-01

We perform three-dimensional dust radiative transfer (RT) calculations on hydrodynamic simulations of isolated and merging disk galaxies in order to quantitatively study the dependence of observed-frame submillimeter (submm) flux density on galaxy properties. We find that submm flux density and star formation rate (SFR) are related in dramatically different ways for quiescently star-forming galaxies and starbursts. Because the stars formed in the merger-induced starburst do not dominate the bolometric luminosity and the rapid drop in dust mass and more compact geometry cause a sharp increase in dust temperature during the burst, starbursts are very inefficient at boosting submm flux density (e.g., a >~ 16 × boost in SFR yields a <~ 2 × boost in submm flux density). Moreover, the ratio of submm flux density to SFR differs significantly between the two modes; thus one cannot assume that the galaxies with highest submm flux density are necessarily those with the highest bolometric luminosity or SFR. These results have important consequences for the bright submillimeter-selected galaxy (SMG) population. Among them are: (1) The SMG population is heterogeneous. In addition to merger-driven starbursts, there is a subpopulation of galaxy pairs, where two disks undergoing a major merger but not yet strongly interacting are blended into one submm source because of the large (gsim 15" or ~130 kpc at z = 2) beam of single-dish submm telescopes. (2) SMGs must be very massive (M sstarf >~ 6 × 1010 M ⊙). (3) The infall phase makes the SMG duty cycle a factor of a few greater than what is expected for a merger-driven starburst. Finally, we provide fitting functions for SCUBA and AzTEC submm flux densities as a function of SFR and dust mass and bolometric luminosity and dust mass; these should be useful for calculating submm flux density in semi-analytic models and cosmological simulations when performing full RT is computationally not feasible.

Experimental Investigation of Shear Driven Liquid Films for Film Cooling Applications in Liquid Rocket Engines

DTIC Science & Technology

2012-12-01

6 1.1.1 Differences Between Hot-Fire at Subcritical Conditions and Cold Flow ........10 1.1.2 Differences at Supercritical Conditions...cooling. 1.1.2 Differences at Supercritical Conditions Liquid film cooling is expected to behave even more differently at supercritical conditions...phase will behave more like the mixing of two gases of dissimilar densities. Once enough heat is imparted into the supercritical fuel film, it

Formation of temperature front in stably stratified turbulence

NASA Astrophysics Data System (ADS)

Kimura, Yoshifumi; Sullivan, Peter; Herring, Jackson

2016-11-01

An important feature of stably stratified turbulence is the significant influence of internal gravity waves which makes stably stratified turbulence unique compared to homogeneous isotropic turbulence. In this paper, we investigate the genesis of temperature fronts-a crucial subject both practically and fundamentally-in stably stratified turbulence using Direct Numerical Simulations (DNS) of the Navier-Stokes equation under the Boussinesq approximation with 10243 grid points. Vertical profiles of temperature fluctuations show almost vertically periodic sawtooth wavy structures with negative and positive layers stacked together with clear boundaries implying a sharp temperature fronts. The sawtooth waves consist of gradual decreasing temperature fluctuations with rapid recovery to a positive value as the frontal boundary is crossed vertically. This asymmetry of gradients comes from the structure that warm temperature region lies on top of cool temperature region, and can be verified in the skewed probability density function (PDF) of vertical temperature gradient. We try to extract the flow structures and mechanism for the formation and maintenance of the strong temperature front numerically.

Evolution of protoplanetary discs with magnetically driven disc winds

NASA Astrophysics Data System (ADS)

Suzuki, Takeru K.; Ogihara, Masahiro; Morbidelli, Alessandro; Crida, Aurélien; Guillot, Tristan

2016-12-01

Aims: We investigate the evolution of protoplanetary discs (PPDs) with magnetically driven disc winds and viscous heating. Methods: We considered an initially massive disc with 0.1 M⊙ to track the evolution from the early stage of PPDs. We solved the time evolution of surface density and temperature by taking into account viscous heating and the loss of mass and angular momentum by the disc winds within the framework of a standard α model for accretion discs. Our model parameters, turbulent viscosity, disc wind mass-loss, and disc wind torque, which were adopted from local magnetohydrodynamical simulations and constrained by the global energetics of the gravitational accretion, largely depends on the physical condition of PPDs, particularly on the evolution of the vertical magnetic flux in weakly ionized PPDs. Results: Although there are still uncertainties concerning the evolution of the vertical magnetic flux that remains, the surface densities show a large variety, depending on the combination of these three parameters, some of which are very different from the surface density expected from the standard accretion. When a PPD is in a wind-driven accretion state with the preserved vertical magnetic field, the radial dependence of the surface density can be positive in the inner region <1-10 au. The mass accretion rates are consistent with observations, even in the very low level of magnetohydrodynamical turbulence. Such a positive radial slope of the surface density strongly affects planet formation because it inhibits the inward drift or even causes the outward drift of pebble- to boulder-sized solid bodies, and it also slows down or even reversed the inward type-I migration of protoplanets. Conclusions: The variety of our calculated PPDs should yield a wide variety of exoplanet systems.

High Energy Density in Azobenzene-based Materials for Photo-Thermal Batteries via Controlled Polymer Architecture and Polymer-Solvent Interactions.

PubMed

Jeong, Seung Pyo; Renna, Lawrence A; Boyle, Connor J; Kwak, Hyunwook S; Harder, Edward; Damm, Wolfgang; Venkataraman, Dhandapani

2017-12-19

Energy densities of ~510 J/g (max: 698 J/g) have been achieved in azobenzene-based syndiotactic-rich poly(methacrylate) polymers. The processing solvent and polymer-solvent interactions are important to achieve morphologically optimal structures for high-energy density materials. This work shows that morphological changes of solid-state syndiotactic polymers, driven by different solvent processings play an important role in controlling the activation energy of Z-E isomerization as well as the shape of the DSC exotherm. Thus, this study shows the crucial role of processing solvents and thin film structure in achieving higher energy densities.

Sexual minority population density and incidence of lung, colorectal and female breast cancer in California.

PubMed

Boehmer, Ulrike; Miao, Xiaopeng; Maxwell, Nancy I; Ozonoff, Al

2014-03-26

Risk factors for breast, colorectal, and lung cancer are known to be more common among lesbian, gay, and bisexual (LGB) individuals, suggesting they may be more likely to develop these cancers. Our objective was to determine differences in cancer incidence by sexual orientation, using sexual orientation data aggregated at the county level. Data on cancer incidence were obtained from the California Cancer Registry and data on sexual orientation were obtained from the California Health Interview Survey, from which a measure of age-specific LGB population density by county was calculated. Using multivariable Poisson regression models, the association between the age-race-stratified incident rate of breast, lung and colorectal cancer in each county and LGB population density was examined, with race, age group and poverty as covariates. Among men, bisexual population density was associated with lower incidence of lung cancer and with higher incidence of colorectal cancer. Among women, lesbian population density was associated with lower incidence of lung and colorectal cancer and with higher incidence of breast cancer; bisexual population density was associated with higher incidence of lung and colorectal cancer and with lower incidence of breast cancer. These study findings clearly document links between county-level LGB population density and cancer incidence, illuminating an important public health disparity.

Channeling, channel density and mass recovery in aquifer transport, with application to the MADE experiment

NASA Astrophysics Data System (ADS)

Fiori, A.

2014-12-01

Channeling effects in heterogeneous formations are studied through a new quantity denoted as channel density a(x,t). Focusing on advection only, a(x,t) is defined as the relative number of streamtubes (or channels) containing solute between x and x + dx at a given time t, regardless of the mass that they carry. The channel density generally differs from the widely employed longitudinal mass distribution m(x,t), and their difference increases with time and the degree of heterogeneity. The difference between a and m reflects the nonuniformity of mass distribution relative to the plume geometry. In particular, the "fast" channels typically carry a larger fraction of mass than their share in their relative volume, which in turn can be rather small. Detecting such channels by a network of monitoring wells may be a challenging task, which might explain the poor solute recovery of some field experiments at increasing times. After application of the proposed concepts to the simple case of stratified formations, we model the channel density and mass distribution pertaining to the MADE experiment, which exhibited poor mass recovery at large times. The results presented in this study emphasize the possible channeling effects at MADE and the general difficulty in sampling the leading edge of the plume, which in turn may contain a significant fraction of the plume mass.

Stability of viscosity stratified flows down an incline: Role of miscibility and wall slip

NASA Astrophysics Data System (ADS)

Ghosh, Sukhendu; Usha, R.

2016-10-01

The effects of wall velocity slip on the linear stability of a gravity-driven miscible two-fluid flow down an incline are examined. The fluids have the matched density but different viscosity. A smooth viscosity stratification is achieved due to the presence of a thin mixed layer between the fluids. The results show that the presence of slip exhibits a promise for stabilizing the miscible flow system by raising the critical Reynolds number at the onset and decreasing the bandwidth of unstable wave numbers beyond the threshold of the dominant instability. This is different from its role in the case of a single fluid down a slippery substrate where slip destabilizes the flow system at the onset. Though the stability properties are analogous to the same flow system down a rigid substrate, slip is shown to delay the surface mode instability for any viscosity contrast. It has a damping/promoting effect on the overlap modes (which exist due to the overlap of critical layer of dominant disturbance with the mixed layer) when the mixed layer is away/close from/to the slippery inclined wall. The trend of slip effect is influenced by the location of the mixed layer, the location of more viscous fluid, and the mass diffusivity of the two fluids. The stabilizing characteristics of slip can be favourably used to suppress the non-linear breakdown which may happen due to the coexistence of the unstable modes in a flow over a substrate with no slip. The results of the present study suggest that it is desirable to design a slippery surface with appropriate slip sensitivity in order to meet a particular need for a specific application.

shock driven instability of a multi-phase particle-gas system

NASA Astrophysics Data System (ADS)

McFarland, Jacob; Black, Wolfgang; Dahal, Jeevan; Morgan, Brandon

2015-11-01

A computational study of a shock driven instability of a multiphse particle-gas system is presented. This instability can evolve in a similar fashion to the Richtmyer-Meshkov (RM) instability, but has addition parameters to be considered. Particle relaxation times, and density differences of the gas and particle-gas system can be adjusted to produce results which are different from the classical RM instability. We will show simulation results from the Ares code, developed at Lawrence Livermore National Laboratory, which uses a particle-in-cell approach to study the effects of the particle-gas system parameters. Mixing parameters will be presented to highlight the suppression of circulation and gas mixing by the particle phase.

Strongly Stratified Turbulence Wakes and Mixing Produced by Fractal Wakes

NASA Astrophysics Data System (ADS)

Dimitrieva, Natalia; Redondo, Jose Manuel; Chashechkin, Yuli; Fraunie, Philippe; Velascos, David

2017-04-01

This paper describes Shliering and Shadowgraph experiments of the wake induced mixing produced by tranversing a vertical or horizontal fractal grid through the interfase between two miscible fluids at low Atwood and Reynolds numbers. This is a configuration design to models the mixing across isopycnals in stably-stratified flows in many environmental relevant situations (either in the atmosphere or in the ocean. The initial unstable stratification is characterized by a reduced gravity: g' = gΔρ ρ where g is gravity, Δρ being the initial density step and ρ the reference density. Here the Atwood number is A = g' _ 2 g . The topology of the fractal wake within the strong stratification, and the internal wave field produces both a turbulent cascade and a wave cascade, with frecuen parametric resonances, the envelope of the mixing front is found to follow a complex non steady 3rd order polinomial function with a maximum at about 4-5 Brunt-Vaisalla non-dimensional time scales: t/N δ = c1(t/N) + c2g Δρ ρ (t/N)2 -c3(t/N)3. Conductivity probes and Shliering and Shadowgraph visual techniques, including CIV with (Laser induced fluorescence and digitization of the light attenuation across the tank) are used in order to investigate the density gradients and the three-dimensionality of the expanding and contracting wake. Fractal analysis is also used in order to estimate the fastest and slowest growing wavelengths. The large scale structures are observed to increase in wave-length as the mixing progresses, and the processes involved in this increase in scale are also examined.Measurements of the pointwise and horizontally averaged concentrations confirm the picture obtained from past flow visualization studies. They show that the fluid passes through the mixing region with relatively small amounts of molecular mixing,and the molecular effects only dominate on longer time scales when the small scales have penetrated through the large scale structures. The Non-stationary dynamicss and structure of stratified fluid flows around a wedge were also studied based of the fundamental equations set using numerical modeling. Due to breaking of naturally existing background diffusion flux of stratifying agent by an impermeable surface of the wedge a complex multi-level vortex system of compensatory fluid motions is formed around the obstacle. The flow is characterized by a wide range of values of internal scales that are absent in a homogeneous liquid. Numerical solution of the fundamental system with the boundary conditions is constructed using a solver such as stratifiedFoam developed within the frame of the open source computational package OpenFOAM using the finite volume method. The computations were performed in parallel using computing resources of the Scientific Research Supercomputer Complex of MSU (SRCC MSU) and the technological platform UniHUB. The evolution of the flow pattern of the wedge by stratified flow has been demonstrated. The complex structure of the fields of physical quantities and their gradients has been shown. Observed in experiment are multiple flow components, including upstream disturbances, internal waves and the downstream wake with submerged transient vortices well reproduced. Structural elements of flow differ in size and laws of variation in space and time. Rich fine flow structure visualized in vicinity and far from the obstacle. The global efficiency of the mixing process is measured and compared with previous estimates of mixing efficiency.

Alcohol prevention on college campuses: the moderating effect of the alcohol environment on the effectiveness of social norms marketing campaigns.

PubMed

Scribner, Richard A; Theall, Katherine P; Mason, Karen; Simonsen, Neal; Schneider, Shari Kessel; Towvim, Laura Gomberg; DeJong, William

2011-03-01

Evaluations of social norms marketing campaigns to reduce college student drinking have produced conflicting results. This study examines whether the effectiveness of such campaigns may be moderated by on-premise alcohol outlet density in the surrounding community. Multilevel analyses were conducted of student survey responses (N= 19,838) from 32 U.S. colleges that took part in one of two 4-year randomized, controlled trials completed for the Social Norms Marketing Research Project (SNMRP). In the models, students by year were nested within treatment (n = 16) and control group (n = 16) campuses, which were characterized by the on-premise outlet density in their surrounding community. The moderating effect of outlet density was introduced into the models as an interaction between the treatment effect (i.e., the effect of the social norms marketing campaigns over time) and outlet density. The models were also stratified by campus alcohol outlet density (high vs. low) to examine the effect of the intervention in each type of setting. There was a significant interaction between the treatment effect and on-premise alcohol outlet density for one of the drinking outcomes targeted by the SNMRP intervention, the number of drinks when partying, and marginal evidence of interaction effects for two other outcomes, maximum recent consumption and a composite drinking scale. In stratified analyses, an intervention effect was observed for three of the four outcomes among students from campuses with lower on-premise alcohol outlet density, whereas no intervention effect was observed among students from campuses with higher on-premise alcohol outlet density. The findings suggest that the campus alcohol environment moderates the effect of social norms marketing interventions. Social norms marketing intervention may be less effective on campuses with higher densities of on-sale alcohol outlets.

Experimental characterization of 3-dimensional gravity-driven fingering in a porous medium

NASA Astrophysics Data System (ADS)

Dalbe, Marie-Julie; Juanes, Ruben

2017-11-01

When water infiltrates a dry porous media, a gravity-driven instability can be observed. Water will penetrate the porous media along preferential paths, called fingers. This gravity-driven unstable multiphase flow has important implications for natural phenomena such as rainwater infiltration in soil and secondary oil migration in reservoir rocks. While several experimental and numerical studies have described the instability in 2-dimensional (2D) settings, fundamental questions remain on the morphodynamics of gravity fingering in 3D. We developed a 3D experimental set-up based on planar laser-induced fluorescence of index-matched fluids that allows us to image this phenomenon dynamically. We study the impact of some crucial parameters such as rainfall rate or grain size on the finger size and velocity. In addition, experiments in stratified media reveal interesting dynamics of finger flow across material interfaces, an essential aspect towards the understanding of water infiltration in soils.

Single-Mode, Supersonic Kelvin-Helmholtz Instability Experiment on OMEGA-EP

NASA Astrophysics Data System (ADS)

Wan, Wesley; Malamud, G.; Di Stefano, C.; Kuranz, C. C.; Drake, R.

2013-06-01

Laboratory laser experiments are able to produce and study phenomena that occur in astrophysical systems, allowing us to study mechanisms relevant to the formation, interaction, and destruction processes of stars and planets. These dynamic processes are strongly affected by hydrodynamic instabilities such as the Kelvin-Helmholtz instability, which arises when shear flow at an interface causes mixing between fluid layers. This instability is commonly observed at the boundary of cloud bands among gas planets, and can act as an atmospheric loss mechanism on planets with little to no intrinsic magnetic field. It is also observed in simulations of astrophysical systems including supernovae and wind-driven clumps. This poster discusses an upcoming experiment for the OMEGA-EP system that will produce a supersonic Kelvin-Helmholtz instability in the high-energy-density regime. This experiment will use a long laser pulse to create a sustained shock through two stratified layers separated by a seeded, single-mode perturbation. A high Mach number is believed to suppress the growth of the Kelvin-Helmholtz instability and, if sufficiently high, prevent growth entirely. We will be quantifying these effects using x-ray radiography. This work is funded by the NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasmas, grant number DE-FG52-09NA29548, and by the National Laser User Facility Program, grant number DE-NA0000850, with additional support provided under Cooperative Agreement No. DE-FC52-08NA28302 through the Laboratory for Laser Energetics, University of Rochester.

ON THE LAUNCHING AND STRUCTURE OF RADIATIVELY DRIVEN WINDS IN WOLF–RAYET STARS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ro, Stephen; Matzner, Christopher D., E-mail: ro@astro.utoronto.ca

Hydrostatic models of Wolf–Rayet (WR) stars typically contain low-density outer envelopes that inflate the stellar radii by a factor of several and are capped by a denser shell of gas. Inflated envelopes and density inversions are hallmarks of envelopes that become super-Eddington as they cross the iron-group opacity peak, but these features disappear when mass loss is sufficiently rapid. We re-examine the structures of steady, spherically symmetric wind solutions that cross a sonic point at high optical depth, identifying the physical mechanism through which the outflow affects the stellar structure, and provide an improved analytical estimate for the critical mass-lossmore » rate above which extended structures are erased. Weak-flow solutions below this limit resemble hydrostatic stars even in supersonic zones; however, we infer that these fail to successfully launch optically thick winds. WR envelopes will therefore likely correspond to the strong, compact solutions. We also find that wind solutions with negligible gas pressure are stably stratified at and below the sonic point. This implies that convection is not the source of variability in WR stars, as has been suggested; however, acoustic instabilities provide an alternative explanation. Our solutions are limited to high optical depths by our neglect of Doppler enhancements to the opacity, and do not account for acoustic instabilities at high Eddington factors; yet, they do provide useful insights into WR stellar structures.« less

Effect of temperature-driven phase transition on energy-storage and -release properties of Pb0.97La0.02[Zr0.55Sn0.30Ti0.15]O3 ceramics

NASA Astrophysics Data System (ADS)

Xu, Ran; Tian, Jingjing; Zhu, Qingshan; Feng, Yujun; Wei, Xiaoyong; Xu, Zhuo

2017-07-01

Temperature-driven phase transition of Pb0.97La0.02[Zr0.55Sn0.30Ti0.15]O3 ceramics was studied, and the consecutive ferroelectric-antiferroelectric-paraelectric (FE-AFE-PE) switching was confirmed. The materials have better dielectric tunability (-82% to 50%) in the AFE state than in the FE state. Also, the phase transition influences the energy-storage and -release performance significantly. A sharp increase in releasable energy density and efficiency was observed due to the temperature-driven FE-AFE transition. Highest releasable energy density, current density, and peak power density were achieved at 130 °C, which was attributed to the highest backward transition field. The stored charge was released completely in AFE and PE states in the microseconds scale, while only a small part of it was released in the FE state. The above results indicate the huge impact of temperature-driven phase transition on dielectrics' performance, which is significant when developing AFE materials working in a wide temperature range.

Turbulence observations in the Gulf of Trieste under moderate wind forcing and different water column stratification

NASA Astrophysics Data System (ADS)

Marcello Falcieri, Francesco; Kantha, Lakshmi; Benetazzo, Alvise; Bergamasco, Andrea; Bonaldo, Davide; Barbariol, Francesco; Malačič, Vlado; Sclavo, Mauro; Carniel, Sandro

2016-03-01

The oceanographic campaign CARPET2014 (Characterizing Adriatic Region Preconditionig EvenTs), (30 January-4 February 2014) collected the very first turbulence data in the Gulf of Trieste (northern Adriatic Sea) under moderate wind (average wind speed 10 m s-1) and heat flux (net negative heat flux ranging from 150 to 400 W m-2). Observations consisted of 38 CTD (Conductivity, Temperature, Depth) casts and 478 microstructure profiles (grouped into 145 ensembles) with three sets of yoyo casts, each lasting for about 12 consecutive hours. Averaging closely repeated casts, such as the ensembles, can lead to a smearing effect when in the presence of a vertical density structure with strong interfaces that can move up or down between subsequent casts under the influence of tides and internal waves. In order to minimize the smearing effect of such displacements on mean quantities, we developed an algorithm to realign successive microstructure profiles to produce sharper and more meaningful mean profiles of measured turbulence parameters. During the campaign, the water column in the gulf evolved from well-mixed to stratified conditions due to Adriatic waters intruding at the bottom along the gulf's south-eastern coast. We show that during the warm and relatively dry winter, the water column in the Gulf of Trieste, even under moderate wind forcing, was not completely mixed due to the influence of bottom waters intruding from the open sea. Inside the gulf, two types of water intrusions were found during yoyo casts: one coming from the northern coast of the Adriatic Sea (i.e. cooler, fresher and more turbid) and one coming from the open sea in front of the Po Delta (i.e. warmer, saltier and less turbid). The two intrusions had different impacts on turbulence kinetic energy dissipation rate profiles. The former, with high turbidity, acted as a barrier to wind-driven turbulence, while the latter, with low sediment concentrations and a smaller vertical density gradient, was not able to suppress downward penetration of turbulence from the surface.

Dust settling in magnetorotationally driven turbulent discs - I. Numerical methods and evidence for a vigorous streaming instability

NASA Astrophysics Data System (ADS)

Balsara, Dinshaw S.; Tilley, David A.; Rettig, Terrence; Brittain, Sean D.

2009-07-01

In this paper, we have used the RIEMANN code for computational astrophysics to study the interaction of a realistic distribution of dust grains with gas at specific radial locations in a vertically stratified protostellar accretion disc. The disc was modelled to have the density and temperature of a minimum mass solar nebula, and shearing box simulations at radii of 0.3 and 10 au are reported here. The disc was driven to a fully developed turbulence via the magnetorotational instability (MRI). The simulations span three gas scaleheights about the disc's midplane. We find that the inclusion of standard dust-to-gas ratios does not have any significant effect on the MRI even when the dust sediments to the midplane of the accretion disc. The density distribution of the dust of all sizes reached a Gaussian profile within two scaleheights of the disc's midplane. The vertical scaleheights of these Gaussian profiles are shown to be proportional to the reciprocal of the square root of the dust radius when large spherical dust grains are considered. This result is consistent with theoretical expectation. The largest two families of dust in one of our simulations show a strong tendency to settle to the midplane of the accretion disc. The large dust tends to organize itself into elongated clumps of high density. The dynamics of these clumps is shown to be consistent with a streaming instability. The streaming instability is seen to be very vigorous and persistent once it forms. Each stream of high-density dust displays a reduced rms velocity dispersion. The velocity directions within the streams are also aligned relative to the mean shear, providing further evidence that we are witnessing a streaming instability. The densest clumpings of large dust are shown to form where the streams intersect. We have also shown that the mean free path and collision time for dust that participates in the streaming instability are reduced by almost two orders of magnitude relative to the average mean free paths and collision times. The rms velocities between the grains also need to fall below a minimum threshold in order for the grains to stick and we show that a small amount of the large dust in our 10 au simulation should have a propensity for grain coalescence. The results of our simulations are likely to be useful for those who model spectral energy distributions of protostellar discs and also for those who model dust coagulation and growth.

Nonadiabatic effects in periodically driven dissipative open quantum systems

NASA Astrophysics Data System (ADS)

Reimer, Viktor; Pedersen, Kim G. L.; Tanger, Niklas; Pletyukhov, Mikhail; Gritsev, Vladimir

2018-04-01

We present a general method to calculate the periodic steady state of a driven-dissipative system coupled to a transmission line (and more generally, to a reservoir) under periodic modulation of its parameters. Using Floquet's theorem, we formulate the differential equation for the system's density operator which has to be solved for a single period of modulation. On this basis we also provide systematic expansions in both the adiabatic and high-frequency regime. Applying our method to three different systems—two- and three-level models as well as the driven nonlinear cavity—we propose periodic modulation protocols of parameters leading to a temporary suppression of effective dissipation rates, and study the arising nonadiabatic features in the response of these systems.

Model Simulation of Diurnal Vertical Migration Patterns of Different-Sized Colonies of Microcystis Employing a Particle Trajectory Approach.

PubMed

Chien, Yu Ching; Wu, Shian Chee; Chen, Wan Ching; Chou, Chih Chung

2013-04-01

Microcystis , a genus of potentially harmful cyanobacteria, is known to proliferate in stratified freshwaters due to its capability to change cell density and regulate buoyancy. In this study, a trajectory model was developed to simulate the cell density change and spatial distribution of Microcystis cells with nonuniform colony sizes. Simulations showed that larger colonies migrate to the near-surface water layer during the night to effectively capture irradiation and become heavy enough to sink during daytime. Smaller-sized colonies instead took a longer time to get to the surface. Simulation of the diurnally varying Microcystis population profile matched the observed pattern in the field when the radii of the multisized colonies were in a beta distribution. This modeling approach is able to take into account the history of cells by keeping track of their positions and properties, such as cell density and the sizes of colonies. It also serves as the basis for further developmental modeling of phytoplanktons that are forming colonies and changing buoyancy.

Data-driven Inference and Investigation of Thermosphere Dynamics and Variations

NASA Astrophysics Data System (ADS)

Mehta, P. M.; Linares, R.

2017-12-01

This paper presents a methodology for data-driven inference and investigation of thermosphere dynamics and variations. The approach uses data-driven modal analysis to extract the most energetic modes of variations for neutral thermospheric species using proper orthogonal decomposition, where the time-independent modes or basis represent the dynamics and the time-depedent coefficients or amplitudes represent the model parameters. The data-driven modal analysis approach combined with sparse, discrete observations is used to infer amplitues for the dynamic modes and to calibrate the energy content of the system. In this work, two different data-types, namely the number density measurements from TIMED/GUVI and the mass density measurements from CHAMP/GRACE are simultaneously ingested for an accurate and self-consistent specification of the thermosphere. The assimilation process is achieved with a non-linear least squares solver and allows estimation/tuning of the model parameters or amplitudes rather than the driver. In this work, we use the Naval Research Lab's MSIS model to derive the most energetic modes for six different species, He, O, N2, O2, H, and N. We examine the dominant drivers of variations for helium in MSIS and observe that seasonal latitudinal variation accounts for about 80% of the dynamic energy with a strong preference of helium for the winter hemisphere. We also observe enhanced helium presence near the poles at GRACE altitudes during periods of low solar activity (Feb 2007) as previously deduced. We will also examine the storm-time response of helium derived from observations. The results are expected to be useful in tuning/calibration of the physics-based models.

Aspects of jamming in two-dimensional athermal frictionless systems.

PubMed

Reichhardt, C; Reichhardt, C J Olson

2014-05-07

In this work we provide an overview of jamming transitions in two dimensional systems focusing on the limit of frictionless particle interactions in the absence of thermal fluctuations. We first discuss jamming in systems with short range repulsive interactions, where the onset of jamming occurs at a critical packing density and where certain quantities show a divergence indicative of critical behavior. We describe how aspects of the dynamics change as the jamming density is approached and how these dynamics can be explored using externally driven probes. Different particle shapes can produce jamming densities much lower than those observed for disk-shaped particles, and we show how jamming exhibits fragility for some shapes while for other shapes this is absent. Next we describe the effects of long range interactions and jamming behavior in systems such as charged colloids, vortices in type-II superconductors, and dislocations. We consider the effect of adding obstacles to frictionless jamming systems and discuss connections between this type of jamming and systems that exhibit depinning transitions. Finally, we discuss open questions such as whether the jamming transition in all these different systems can be described by the same or a small subset of universal behaviors, as well as future directions for studies of jamming transitions in two dimensional systems, such as jamming in self-driven or active matter systems.

Interaction of mammographic breast density with menopausal status and postmenopausal hormone use in relation to the risk of aggressive breast cancer subtypes.

PubMed

Yaghjyan, Lusine; Tamimi, Rulla M; Bertrand, Kimberly A; Scott, Christopher G; Jensen, Matthew R; Pankratz, V Shane; Brandt, Kathy; Visscher, Daniel; Norman, Aaron; Couch, Fergus; Shepherd, John; Fan, Bo; Chen, Yunn-Yi; Ma, Lin; Beck, Andrew H; Cummings, Steven R; Kerlikowske, Karla; Vachon, Celine M

2017-09-01

We examined the associations of mammographic breast density with breast cancer risk by tumor aggressiveness and by menopausal status and current postmenopausal hormone therapy. This study included 2596 invasive breast cancer cases and 4059 controls selected from participants of four nested case-control studies within four established cohorts: the Mayo Mammography Health Study, the Nurses' Health Study, Nurses' Health Study II, and San Francisco Mammography Registry. Percent breast density (PD), absolute dense (DA), and non-dense areas (NDA) were assessed from digitized film-screen mammograms using a computer-assisted threshold technique and standardized across studies. We used polytomous logistic regression to quantify the associations of breast density with breast cancer risk by tumor aggressiveness (defined as presence of at least two of the following tumor characteristics: size ≥2 cm, grade 2/3, ER-negative status, or positive nodes), stratified by menopausal status and current hormone therapy. Overall, the positive association of PD and borderline inverse association of NDA with breast cancer risk was stronger in aggressive vs. non-aggressive tumors (≥51 vs. 11-25% OR 2.50, 95% CI 1.94-3.22 vs. OR 2.03, 95% CI 1.70-2.43, p-heterogeneity = 0.03; NDA 4th vs. 2nd quartile OR 0.54, 95% CI 0.41-0.70 vs. OR 0.71, 95% CI 0.59-0.85, p-heterogeneity = 0.07). However, there were no differences in the association of DA with breast cancer by aggressive status. In the stratified analysis, there was also evidence of a stronger association of PD and NDA with aggressive tumors among postmenopausal women and, in particular, current estrogen+progesterone users (≥51 vs. 11-25% OR 3.24, 95% CI 1.75-6.00 vs. OR 1.93, 95% CI 1.25-2.98, p-heterogeneity = 0.01; NDA 4th vs. 2nd quartile OR 0.43, 95% CI 0.21-0.85 vs. OR 0.56, 95% CI 0.35-0.89, p-heterogeneity = 0.01), even though the interaction was not significant. Our findings suggest that associations of mammographic density with breast cancer risk differ by tumor aggressiveness. While there was no strong evidence that these associations differed by menopausal status or hormone therapy, they did appear more prominent among current estrogen+progesterone users.

Biological and Genomic Differences of ERG Oncoprotein-Stratified Prostate Cancers from African and Caucasian Americans

DTIC Science & Technology

2015-10-01

AWARD NUMBER: W81XWH-13-2-0096 TITLE: Biological and Genomic Differences of ERG Oncoprotein-Stratified Prostate Cancers from African and...Biological and Genomic Differences of ERG Oncoprotein-Stratified Prostate Cancers from African and Caucasian Americans Sb. GRANT NUMBER W81 XWH-13-2...differences in the distribution of key clinico-pathologic patient features and molecular determinants for both ERG positive and ERG negative prostate cancer

Ion acceleration in electrostatic collisionless shock: on the optimal density profile for quasi-monoenergetic beams

NASA Astrophysics Data System (ADS)

Boella, E.; Fiúza, F.; Stockem Novo, A.; Fonseca, R.; Silva, L. O.

2018-03-01

A numerical study on ion acceleration in electrostatic shock waves is presented, with the aim of determining the best plasma configuration to achieve quasi-monoenergetic ion beams in laser-driven systems. It was recently shown that tailored near-critical density plasmas characterized by a long-scale decreasing rear density profile lead to beams with low energy spread (Fiúza et al 2012 Phys. Rev. Lett. 109 215001). In this work, a detailed parameter scan investigating different plasma scale lengths is carried out. As result, the optimal plasma spatial scale length that allows for minimizing the energy spread while ensuring a significant reflection of ions by the shock is identified. Furthermore, a new configuration where the required profile has been obtained by coupling micro layers of different densities is proposed. Results show that this new engineered approach is a valid alternative, guaranteeing a low energy spread with a higher level of controllability.

What causes the buoyancy reversal in compressible convection?

NASA Technical Reports Server (NTRS)

Chan, K. L.

1983-01-01

The problem posed by the existence of a negative buoyancy work region at the top of cellular type convection in a deeply stratified superadiabatic layer (Massaguer and Zahn, 1980) is addressed. It is approached by studying two-dimensional cellular compressible convection with different physical parameters. The results suggest that a large viscosity, together with density stratification, is responsible for the buoyancy reversal. The numerical results obtained are analyzed. It is pointed out, however, that in an astrophysical situation a fluid involved in convection will generally have very small viscosity. It is therefore thought unlikely that buoyancy reversal occurs in this way.

Baroclinic Vortices in Rotating Stratified Shearing Flows: Cyclones, Anticyclones, and Zombie Vortices

NASA Astrophysics Data System (ADS)

Hassanzadeh, Pedram

Large coherent vortices are abundant in geophysical and astrophysical flows. They play significant roles in the Earth's oceans and atmosphere, the atmosphere of gas giants, such as Jupiter, and the protoplanetary disks around forming stars. These vortices are essentially three-dimensional (3D) and baroclinic, and their dynamics are strongly influenced by the rotation and density stratification of their environments. This work focuses on improving our understanding of the physics of 3D baroclinic vortices in rotating and continuously stratified flows using 3D spectral simulations of the Boussinesq equations, as well as simplified mathematical models. The first chapter discusses the big picture and summarizes the results of this work. In Chapter 2, we derive a relationship for the aspect ratio (i.e., vertical half-thickness over horizontal length scale) of steady and slowly-evolving baroclinic vortices in rotating stratified fluids. We show that the aspect ratio is a function of the Brunt-Vaisala frequencies within the vortex and outside the vortex, the Coriolis parameter, and the Rossby number of the vortex. This equation is basically the gradient-wind equation integrated over the vortex, and is significantly different from the previously proposed scaling laws that find the aspect ratio to be only a function of the properties of the background flow, and independent of the dynamics of the vortex. Our relation is valid for cyclones and anticyclones in either the cyclostrophic or geostrophic regimes; it works with vortices in Boussinesq fluids or ideal gases, and non-uniform background density gradient. The relation for the aspect ratio has many consequences for quasi-equilibrium vortices in rotating stratified flows. For example, cyclones must have interiors more stratified than the background flow (i.e., super-stratified), and weak anticyclones must have interiors less stratified than the background (i.e., sub-stratified). In addition, this equation is useful to infer the height and internal stratification of some astrophysical and geophysical vortices because direct measurements of their vertical structures are difficult. In Chapter 3, we show numerically and experimentally that localized suction in rotating continuously stratified flows produces three-dimensional baroclinic cyclones. As expected from Chapter 2, the interiors of these cyclones are super-stratified. Suction, modeled as a small spherical sink in the simulations, creates an anisotropic flow toward the sink with directional dependence changing with the ratio of the Coriolis parameter to the Brunt-Vaisala frequency. Around the sink, this flow generates cyclonic vorticity and deflects isopycnals so that the interior of the cyclone becomes super-stratified. The super-stratified region is visualized in the companion experiments that we helped to design and analyze using the synthetic schlieren technique. Once the suction stops, the cyclones decay due to viscous dissipation in the simulations and experiments. The numerical results show that the vertical velocity of viscously decaying cyclones flows away from the cyclone's midplane, while the radial velocity flows toward the cyclone's center. This observation is explained based on the cyclo-geostrophic balance. This vertical velocity mixes the flow inside and outside of cyclone and reduces the super-stratification. We speculate that the predominance of anticyclones in geophysical and astrophysical flows is due to the fact that anticyclones require sub-stratification, which occurs naturally by mixing, while cyclones require super-stratification. In Chapter 4, we show that a previously unknown instability creates space-filling lattices of 3D turbulent baroclinic vortices in linearly-stable, rotating, stratified shear flows. The instability starts from a newly discovered family of easily-excited critical layers. This new family, named the baroclinic critical layer, has singular vertical velocities; the traditional family of (barotropic) critical layer has singular stream-wise velocities and is hard to excite. In our simulations, the baroclinic critical layers in rotating stably-stratified linear shear are excited by small-volume, small-amplitude vortices or waves. The excited baroclinic critical layers then intensify by drawing energy from the background shear and roll-up into large coherent 3D vortices that excite new critical layers and vortices. The vortices self-similarly replicate to create lattices of turbulent vortices. These vortices persist for all time and are called zombie vortices because they can occur in the dead zones of protoplanetary disks. The self-replication of zombie vortices can de-stabilize the otherwise linearly and finite-amplitude stable Keplerian shear and lead to the formation of stars and planets. (Abstract shortened by UMI.)

Characterization of forced response of density stratified reacting wake

NASA Astrophysics Data System (ADS)

Pawar, Samadhan A.; Sujith, Raman I.; Emerson, Benjamin; Lieuwen, Tim

2018-02-01

The hydrodynamic stability of a reacting wake depends primarily on the density ratio [i.e., ratio of unburnt gas density (ρu) to burnt gas density (ρb)] of the flow across the wake. The variation of the density ratio from high to low value, keeping ρ u / ρ b > 1 , transitions dynamical characteristics of the reacting wake from a linearly globally stable (or convectively unstable) to a globally unstable mode. In this paper, we propose a framework to analyze the effect of harmonic forcing on the deterministic and synchronization characteristics of reacting wakes. Using the recurrence quantification analysis of the forced wake response, we show that the deterministic behaviour of the reacting wake increases as the amplitude of forcing is increased. Furthermore, for different density ratios, we found that the synchronization of the top and bottom branches of the wake with the forcing signal is dependent on whether the mean frequency of the natural oscillations of the wake (fn) is lesser or greater than the frequency of external forcing (ff). We notice that the response of both branches (top and bottom) of the reacting wake to the external forcing is asymmetric and symmetric for the low and high density ratios, respectively. Furthermore, we characterize the phase-locking behaviour between the top and bottom branches of the wake for different values of density ratios. We observe that an increase in the density ratio results in a gradual decrease in the relative phase angle between the top and bottom branches of the wake, which leads to a change in the vortex shedding pattern from a sinuous (anti-phase) to a varicose (in-phase) mode of the oscillations.

Observation of parametric instabilities in the quarter critical density region driven by the Nike KrF laser

NASA Astrophysics Data System (ADS)

Weaver, J. L.; Oh, J.; Phillips, L.; Afeyan, B.; Seely, J.; Kehne, D.; Brown, C. M.; Obenschain, S. P.; Serlin, V.; Schmitt, A. J.; Feldman, U.; Lehmberg, R. H.; Mclean, E.; Manka, C.

2013-02-01

The krypton-fluoride (KrF) laser is an attractive choice for inertial confinement fusion due to its combination of short wavelength (λ =248 nm), large bandwidth (up to 3 THz), and superior beam smoothing by induced spatial incoherence. These qualities improve the overall hydrodynamics of directly driven pellet implosions and should allow use of increased laser intensity due to higher thresholds for laser plasma instabilities when compared to frequency tripled Nd:glass lasers (λ =351 nm). Here, we report the first observations of the two-plasmon decay instability using a KrF laser. The experiments utilized the Nike laser facility to irradiate solid plastic planar targets over a range of pulse lengths (0.35 ns≤τ≤1.25 ns) and intensities (up to 2×1015 W/cm2). Variation of the laser pulse created different combinations of electron temperature and electron density scale length. The observed onset of instability growth was consistent with the expected scaling that KrF lasers have a higher intensity threshold for instabilities in the quarter critical density region.

Conformity-driven agents support ordered phases in the spatial public goods game

NASA Astrophysics Data System (ADS)

Javarone, Marco Alberto; Antonioni, Alberto; Caravelli, Francesco

2016-05-01

We investigate the spatial Public Goods Game in the presence of fitness-driven and conformity-driven agents. This framework usually considers only the former type of agents, i.e., agents that tend to imitate the strategy of their fittest neighbors. However, whenever we study social systems, the evolution of a population might be affected also by social behaviors as conformism, stubbornness, altruism, and selfishness. Although the term evolution can assume different meanings depending on the considered domain, here it corresponds to the set of processes that lead a system towards an equilibrium or a steady state. We map fitness to the agents' payoff so that richer agents are those most imitated by fitness-driven agents, while conformity-driven agents tend to imitate the strategy assumed by the majority of their neighbors. Numerical simulations aim to identify the nature of the transition, on varying the amount of the relative density of conformity-driven agents in the population, and to study the nature of related equilibria. Remarkably, we find that conformism generally fosters ordered cooperative phases and may also lead to bistable behaviors.

Density profiles around A+B→C reaction-diffusion fronts in partially miscible systems: A general classification.

PubMed

Loodts, V; Trevelyan, P M J; Rongy, L; De Wit, A

2016-10-01

Various spatial density profiles can develop in partially miscible stratifications when a phase A dissolves with a finite solubility into a host phase containing a dissolved reactant B. We investigate theoretically the impact of an A+B→C reaction on such density profiles in the host phase and classify them in a parameter space spanned by the ratios of relative contributions to density and diffusion coefficients of the chemical species. While the density profile is either monotonically increasing or decreasing in the nonreactive case, reactions combined with differential diffusivity can create eight different types of density profiles featuring up to two extrema in density, at the reaction front or below it. We use this framework to predict various possible hydrodynamic instability scenarios inducing buoyancy-driven convection around such reaction fronts when they propagate parallel to the gravity field.

Observed flow compensation associated with the MOC at 26.5 degrees N in the Atlantic.

PubMed

Kanzow, Torsten; Cunningham, Stuart A; Rayner, Darren; Hirschi, Joël J-M; Johns, William E; Baringer, Molly O; Bryden, Harry L; Beal, Lisa M; Meinen, Christopher S; Marotzke, Jochem

2007-08-17

The Atlantic meridional overturning circulation (MOC), which provides one-quarter of the global meridional heat transport, is composed of a number of separate flow components. How changes in the strength of each of those components may affect that of the others has been unclear because of a lack of adequate data. We continuously observed the MOC at 26.5 degrees N for 1 year using end-point measurements of density, bottom pressure, and ocean currents; cable measurements across the Straits of Florida; and wind stress. The different transport components largely compensate for each other, thus confirming the validity of our monitoring approach. The MOC varied over the period of observation by +/-5.7 x 10(6) cubic meters per second, with density-inferred and wind-driven transports contributing equally to it. We find evidence for depth-independent compensation for the wind-driven surface flow.

Accretion-driven turbulence in filaments - I. Non-gravitational accretion

NASA Astrophysics Data System (ADS)

Heigl, S.; Burkert, A.; Gritschneder, M.

2018-03-01

We study accretion-driven turbulence for different inflow velocities in star-forming filaments using the code RAMSES. Filaments are rarely isolated objects and their gravitational potential will lead to radially dominated accretion. In the non-gravitational case, accretion by itself can already provoke non-isotropic, radially dominated turbulent motions responsible for the complex structure and non-thermal line widths observed in filaments. We find that there is a direct linear relation between the absolute value of the total density-weighted velocity dispersion and the infall velocity. The turbulent velocity dispersion in the filaments is independent of sound speed or any net flow along the filament. We show that the density-weighted velocity dispersion acts as an additional pressure term, supporting the filament in hydrostatic equilibrium. Comparing to observations, we find that the projected non-thermal line width variation is generally subsonic independent of inflow velocity.

A Concept of Cross-Ferroic Plasma Turbulence

PubMed Central

Inagaki, S.; Kobayashi, T.; Kosuga, Y.; Itoh, S.-I.; Mitsuzono, T.; Nagashima, Y.; Arakawa, H.; Yamada, T.; Miwa, Y.; Kasuya, N.; Sasaki, M.; Lesur, M.; Fujisawa, A.; Itoh, K.

2016-01-01

The variety of scalar and vector fields in laboratory and nature plasmas is formed by plasma turbulence. Drift-wave fluctuations, driven by density gradients in magnetized plasmas, are known to relax the density gradient while they can generate flows. On the other hand, the sheared flow in the direction of magnetic fields causes Kelvin-Helmholtz type instabilities, which mix particle and momentum. These different types of fluctuations coexist in laboratory and nature, so that the multiple mechanisms for structural formation exist in extremely non-equilibrium plasmas. Here we report the discovery of a new order in plasma turbulence, in which chained structure formation is realized by cross-interaction between inhomogeneities of scalar and vector fields. The concept of cross-ferroic turbulence is developed, and the causal relation in the multiple mechanisms behind structural formation is identified, by measuring the relaxation rate and dissipation power caused by the complex turbulence-driven flux. PMID:26917218

Ultrasonic wave propagation in trabecular bone predicted by the stratified model

NASA Technical Reports Server (NTRS)

Lin, W.; Qin, Y. X.; Rubin, C.

2001-01-01

The objective of this study was to investigate ultrasound propagation in trabecular bone by considering the wave reflection and transmission in a multilayered medium. The use of ultrasound to identify those at risk of osteoporosis is a promising diagnostic method providing a measure of bone mineral density (BMD). A stratified model was proposed to study the effect of transmission and reflection of ultrasound wave within the trabecular architecture on the relationship between ultrasound and BMD. The results demonstrated that ultrasound velocity in trabecular bone was highly correlated with the bone apparent density (r=0.97). Moreover, a consistent pattern of the frequency dependence of ultrasound attenuation coefficient has been observed between simulation using this model and experimental measurement of trabecular bone. The normalized broadband ultrasound attenuation (nBUA) derived from the simulation results revealed that nBUA was nonlinear with respect to trabecular porosity and BMD. The curve of the relationship between nBUA and BMD was parabolic in shape, and the peak magnitude of nBUA was observed at approximately 60% of bone porosity. These results agreed with the published experimental data and demonstrated that according to the stratified model, reflection and transmission were important factors in the ultrasonic propagation through the trabecular bone.

Numerical calculations of non-inductive current driven by microwaves in JET

NASA Astrophysics Data System (ADS)

Kirov, K. K.; Baranov, Yu; Mailloux, J.; Nave, M. F. F.; Contributors, JET

2016-12-01

Recent studies at JET focus on analysis of the lower hybrid (LH) wave power absorption and current drive (CD) calculations by means of a new ray tracing (RT)/Fokker-Planck (FP) package. The RT code works in real 2D geometry accounting for the plasma boundary and the launcher shape. LH waves with different parallel refractive index, {{N}\\parallel} , spectra in poloidal direction can be launched thus simulating authentic antenna spectrum with rows fed by different combinations of klystrons. Various FP solvers were tested most advanced of which is a relativistic bounce averaged FP code. LH wave power deposition profiles from the new RT/FP code were compared to the experimental results from electron cyclotron emission (ECE) analysis of pulses at 3.4 T low and high density. This kind of direct comparison between power deposition profiles from experimental ECE data and numerical model were carried out for the first time for waves in the LH range of frequencies. The results were in a reasonable agreement with experimental data at lower density, line averaged values of {{n}\\text{e}}≈ 2.4× {{10}19} {{\\text{m}}-3} . At higher density, {{n}\\text{e}}≈ 3× {{10}19} {{\\text{m}}-3} , the code predicted larger on-axis LH power deposition, which is inconsistent with the experimental observations. Both calculations were unable to produce LH wave absorption at the plasma periphery, which contradicts to the analysis of the ECE data and possible sources of these discrepancies have been briefly discussed in the paper. The code was also used to calculate the LH power deposition and CD profiles for the low-density preheat phase of JET’s advanced tokamak (AT) scenario. It was found that as the density evolves from hollow to flat and then to a more peaked profile the LH power and driven current move inward i.e. towards the plasma axis. A total driven current of about 70 kA for 1 MW of launched LH power was predicted in these conditions.

Flow of two immiscible fluids in a periodically constricted tube: Transitions to stratified, segmented, churn, spray or segregated flow

NASA Astrophysics Data System (ADS)

Tsamopoulos, John; Fraggedakis, Dimitris; Dimakopoulos, Yiannis

2015-11-01

We study the flow of two immiscible, Newtonian fluids in a periodically constricted tube driven by a constant pressure gradient. Our Volume-of-Fluid algorithm is used to solve the governing equations. First the code is validated by comparing its predictions to previously reported results for stratified and pulsing flow. Then it is used to capture accurately all the significant topological changes that take place. Initially, the fluids have a core-annular arrangement, which is found to either remain the same or change to a different arrangement depending on the fluid properties, the pressure driving the flow or the flow geometry. The flow-patterns that appear are the core-annular, segmented, churn, spray and segregated flow. The predicted scalings near pinching of the core fluid concur with similarity predictions and earlier numerical results (Cohen et al. (1999)). Flow-pattern maps are constructed in terms of the Reynolds and Weber numbers. Our results provide deeper insights in the mechanism of the pattern transitions and are in agreement with previous studies on core-annular flow (Kouris & Tsamopoulos (2001 & 2002)), segmented flow (Lac & Sherwood (2009)) and churn flow (Bai et al. (1992)). GSRT of Greece through the program ``Excellence'' (Grant No. 1918, entitled ``FilCoMicrA'').

[HDL-C/apoA-I]: A multivessel cardiometabolic risk marker in women with T2DM.

PubMed

Hermans, Michel P; Valensi, Paul; Ahn, Sylvie A; Rousseau, Michel F

2018-01-01

Although women have higher high-density lipoprotein cholesterol (HDL-C) than have men, their HDL particles are also prone to become small, dense, and dysfunctional in case of type 2 diabetes mellitus (T2DM). To assess the vascular risk related to HDLs of different sizes/densities without direct measurement, we adjusted HDL-C to its main apolipoprotein (apoA-I) as [HDL-C/apoA-I]. This ratio estimates HDL sizes and provides indices as to their number, cholesterol load, and density. We stratified 280 Caucasian T2DM women according to [HDL-C/apoA-I] quartiles (Q) to determine how they are segregated according to cardiometabolic risk, β-cell function, glycaemic control, and vascular complications. Five parameters were derived from combined determination of HDL-C and apoA-I: HDL size, HDL number, cholesterol load per particle (pP), apoA-I pP, and HDL density. An adverse cardiometabolic profile characterized QI and QII patients whose HDLs were denser and depleted in apoA-I, whereas QIII patients had HDLs with characteristics closer to those of controls. QIV patients had HDLs of supernormal size/composition and a more favourable phenotype in terms of fat distribution; insulin sensitivity (64% vs 41%), metabolic syndrome, and β-cell function (32% vs 23%); exogenous insulin (44 vs 89 U·d -1 ); and glycaemic control (glycated haemoglobin, 56 vs 61 mmol·mol -1 ), associated with lower prevalence of microvascular/macrovascular complications: all-cause microangiopathy 47% vs 61%; retinopathy 22% vs 34%; all-cause macroangiopathy 19% vs 31%; and coronary artery disease 6% vs 24% (P < .05). [HDL-C/apoA-I] can stratify T2DM women according to metabolic phenotype, macrovascular and coronary damage, β-cell function, microangiopathic risk, and retinopathy. This ratio is a versatile and readily available marker of cardiometabolic status and vascular complications in T2DM women. Copyright © 2017 John Wiley & Sons, Ltd.

Theoretical study of the influence of chemical reactions and physical parameters on the convective dissolution of CO2 in aqueous solutions

NASA Astrophysics Data System (ADS)

Loodts, Vanessa; Rongy, Laurence; De Wit, Anne

2014-05-01

Subsurface carbon sequestration has emerged as a promising solution to the problem of increasing atmospheric carbon dioxide (CO2) levels. How does the efficiency of such a sequestration process depend on the physical and chemical characteristics of the storage site? This question is emblematic of the need to better understand the dynamics of CO2 in subsurface formations, and in particular, the properties of the convective dissolution of CO2 in the salt water of aquifers. This dissolution is known to improve the safety of the sequestration by reducing the risks of leaks of CO2 to the atmosphere. Buoyancy-driven convection makes this dissolution faster by transporting dissolved CO2 further away from the interface. Indeed, upon injection, the less dense CO2 phase rises above the aqueous layer where it starts to dissolve. The dissolved CO2 increases the density of the aqueous solution, thereby creating a layer of denser CO2-rich solution above less dense solution. This unstable density gradient in the gravity field is at the origin of convection. In this framework, we theoretically investigate the effect of CO2 pressure, salt concentration, temperature, and chemical reactions on the dissolution-driven convection of CO2 in aqueous solutions. On the basis of a linear stability analysis, we assess the stability of the time-dependent density profiles developing when CO2 dissolves in an aqueous layer below it. We predict that increasing CO2 pressure destabilizes the system with regard to buoyancy-driven convection, because it increases the density gradient at the origin of the instability. By contrast, increasing salt concentration or temperature stabilizes the system via effects on CO2 solubility, solutal expansion coefficient, diffusion coefficient and on the viscosity and density of the solution. We also show that a reaction of CO2 with chemical species dissolved in the aqueous solution can either enhance or decrease the amplitude of the convective dissolution compared to the non reactive one. On the basis of a reaction-diffusion-convection model, we classify the various possible cases and show that the difference between the solutal expansion coefficients of the reactant and of the product governs the type of density profile building up in the aqueous solution and thus the stability of the system. By contrast to non reactive density profiles, reactive density profiles can feature a minimum that induces a delay of the buoyancy-driven convection. This work identifies the parameters that could influence the dissolution-driven convection in the aquifers, and thus impact the safety of the sequestration. In other words, this theoretical study shows that it is crucial to analyse the composition and reactivity of potential storage sites to choose those that will be most efficient for long-term CO2 sequestration.

Nonequilibrium dynamic phases in driven vortex lattices with periodic pinning

NASA Astrophysics Data System (ADS)

Reichhardt, Charles Michael

1998-12-01

We present the results of an extensive series of simulations of flux-gradient and current driven vortices interacting with either random or periodically arranged pinning sites. First, we consider flux-gradient-driven simulations of superconducting vortices interacting with strong randomly-distributed columnar pinning defects, as an external field H(t) is quasi-statically swept from zero through a matching field Bsb{phi}. Here, we find significant changes in the behavior of the local flux density B(x, y, H(t)), magnetization M(H(t)), critical current Jsb{c}(B(t)), and the individual vortex flow paths, as the local flux density crosses Bsb{phi}. Further, we find that for a given pin density, Jsb{c}(B) can be enhanced by maximizing the distance between the pins for B < Bsb{phi}. For the case of periodic pinning sites as a function of applied field, we find a rich variety of ordered and partially-ordered vortex lattice configurations. We present formulas that predict the matching fields at which commensurate vortex configurations occur and the vortex lattice orientation with respect to the pinning lattice. Our results are in excellent agreement with recent imaging experiments on square pinning arrays (K. Harada et al., Science 274, 1167 (1996)). For current driven simulations with periodic pinning we find a remarkable number of dynamical plastic flow phases. Signatures of the transitions between these different dynamical phases include sudden jumps in the current-voltage curves, hysteresis, as well as marked changes in the vortex trajectories and vortex lattice order. These phases are outlined in a series of dynamic phase diagrams. We show that several of these phases and their phase-boundaries can be understood in terms of analytical arguments. Finally, when the vortex lattice is driven at varying angles with respect to the underlying periodic pinning array, the transverse voltage-current V(I) curves show a series of mode-locked plateaus with the overall V(I) forming a devil's staircase structure.

Intermittency and Topology of Shock Induced Mixing

NASA Astrophysics Data System (ADS)

Tellez, Jackson; Redondo, Jose M.; Ben Mahjoub, Otman; Malik, Nadeem; Vila, Teresa

2016-04-01

The advance of a Rayleigh-Taylor front is described in Linden & Redondo (1991),[1-3] and may be shown to follow a quadratic law in time where the width of the growing region of instability depends on the local mixing efficiency of the different density fluids that accelerate against each other g is the acceleration and A is the Atwood number defined as the diference of densities divided by their sum. This results show the independence of the large amplitude structures on the initial conditions the width of the mixing region depends also on the intermittency of the turbulence. Then dimensional analysis may also depend on the relevant reduced acceleration driven time and the molecular reactive time akin to Damkholer number and the fractal structure of the contact zone [2,4]. Detailed experiments and simulations on RT and RM shock induced fronts analized with respect to structure functions are able to determine which mechanisms are most effective in local mixing which increase the effective fractal dimension, as well as the effect of higher order geometrical parameters, such as the structure functions, in non-homogeneous fluids (Mahjoub et al 1998)[5]. The structure of a Mixing blob shows a relatively sharp head with most of the mixing taking place at the sides due to what seems to be shear instability very similar to the Kelvin-Helmholtz instabilities, but with sideways accelerations. The formation of the blobs and spikes with their secondary instabilities produces a turbulent cascade, evident just after about 1 non-dimensional time unit, from a virtual time origin that takes into account the linear growth phase, as can be seen by the growth of the fractal dimension for different volume fractions. Two-dimensional cuts of the 3D flow also show that vortex flows have closed or spiral streamlines around their core. Examples of such flows can be also seen in the laboratory, for example at the interface of atwo-layer stratified fluid in a tank in which case streamlines are more regular. Mixing in turbulent flows remains less well understood, and in spite research some basic problems are still virtually unexplored. Th e indications suggest that mixing in non-decaying and accelerating turbulent flows are different from those in vortical and steady flows. Fluid element pairs separate, neither linearly nor exponentially but according to a generalized intermittent Richardson's law. Fractal analysis in the laboratory shows that fluid element pairs travel close to each other for a long time until they separate quite suddenly suggest that straining regions around hyperbolic points play an important role in the violent turbulent stirring and in the mechanisms by which turbulence causes fluid element pairs to move apart [6,7]. So the eddies that are most effective in separating fluid elements are those that have a size comparable to the instantaneous separation between the two fluid elements. This is seen in both RT and RM instabilities. For a constant acceleration, the RT instability is found to grow self -similarly according to mixing coefficients which when measured over a comprehensive range of density ratio (Atwood nubers)show that the results are found applicable to supernova exlposions.For an impulsive acceleration (RM), there are two components. The RM impulse from a shock is greatly reduced at high Mach number due to compressive effects in reasonable agreement with linear theory. The ensuing motion is essentially incompressible and described by a power law However, the exponents obtained from the compressible RM experiments are larger than those obtained from incompressible RT experiments. The discrepancy is not well understood but intermittency differences could explain the role of compressibility in fractal media. [1] Linden P.F., Redondo J.M. and Youngs D. (1994) Molecular mixing in Rayleigh-Taylor Instability. Jour. Fluid Mech. 265, 97-124. [2] Redondo, J.M., 1990. The structure of density interfaces. Ph.D. Thesis. DAMTP, University of Cambridge. Cambridge [3] Redondo J.M. (1996) Vertical microstructure and mixing in stratified flows. Advances in Turbulence VI. Eds. S. Gavrilakis et al. 605-608. [4] Redondo J.M.,M.A. Sanchez y R. Castilla (2000) Vortical structures in stratified turbulent flows, Turbulent diffusion in the environment. Eds. Redondo J.M. and Babiano A. 113-120. [5] Mahjoub, O. B., Babiano A. and Redondo, J. M.: Structure functions in complex flows, Flow, Turbulence and Combustion, 59,299-313, 1998. [6] Malik, N.A. Vassilicos, J.C. 1999 A Lagrangian model of turbulent dispersion with turbulent-like flow structure: comparison with direct numerical simulation for two-particle statistics. Phys. Fluids, 11, 1572-1580. [7] Fung, J.C.H., Hunt, J.C.R., Malik, N.A. and Perkins, R.J.(1992. Kinematic simulation of homogeneous turbulence by unsteady random Fourier modes. J. Fluid Mech.236-281. [8] Tarquis, A. M., Platonov, A., Matulka, A., Grau, J., Sekula, E., Diez, M., & Redondo, J. M. (2014). Application of multifractal analysis to the study of SAR features and oil spills on the ocean surface. Nonlinear Processes in Geophysics, 21(2), 439-450. [9] Fraunie, P., Berreba, S., Chashechkin, Y. D., Velasco, D., & Redondo, J. M. (2008). Large eddy simulation and laboratory experiments on the decay of grid wakes in strongly stratified flows. Nuovo Cimento C, 31, 909-930.

Construction of photo-driven bioanodes using thylakoid membranes and multi-walled carbon nanotubes.

PubMed

Takeuchi, Ryosuke; Suzuki, Arato; Sakai, Kento; Kitazumi, Yuki; Shirai, Osamu; Kano, Kenji

2018-04-03

A photo-driven bioanode was constructed using the thylakoid membrane from spinach, carbon nanotubes, and an artificial mediator. By considering a linear free-energy relationship in the electron transfer from the thylakoid membrane to the mediators, and the oxygen resistance of the reduced mediators, 1,2-naphthoquinone was selected as the most suitable mediator for the photo-driven bioanode. Water-dispersed multi-walled carbon nanotubes served as scaffolds to hold the thylakoid membrane on a porous electrode. The constructed photo-driven bioanode exhibited a photocurrent density of over 100μAcm -2 at a photon flux density of 1500μmolm -2 s -1 . Copyright © 2018. Published by Elsevier B.V.

Properties of internal solitary waves in a symmetric three-layer fluid

NASA Astrophysics Data System (ADS)

Vladykina, E. A.; Polukhina, O. E.; Kurkin, A. A.

2009-04-01

Though all the natural media have smooth density stratifications (with the exception of special cases such as sea surface, inversion layer in the atmosphere), the scales of density variations can be different, and some of them can be considered as very sharp. Therefore for the description of internal wave propagation and interaction in the ocean and atmosphere the n-layer models are often used. In these models density profile is usually approximated by a piecewise-constant function. The advantage of the layered models is the finite number of parameters and relatively simple solutions of linear and weakly nonlinear problems. Layered models are also very popular in the laboratory experiments with stratified fluid. In this study we consider symmetric, continuously stratified, smoothed three-layer fluid bounded by rigid horizontal surface and bottom. Three-layer stratification is proved to be a proper approximation of sea water density profile in some basins in the World Ocean with specific hydrological conditions. Such a medium is interesting from the point of view of internal gravity wave dynamics, because in the symmetric case it leads to disappearing of quadratic nonlinearity when described in the framework of weakly nonlinear evolutionary models, that are derived through the asymptotic expansion in small parameters of nonlinearity and dispersion. The goal of our study is to determine the properties of localized stationary internal gravity waveforms (solitary waves) in this symmetric three-layer fluid. The investigation is carried out in the framework of improved mathematical model describing the transformation of internal wave fields generated by an initial disturbance. The model is based on the program complex for the numerical simulation of the two-dimensional (vertical plane) fully nonlinear Euler equations for incompressible stratified fluid under the Boussinesq approximation. Initial disturbances of both polarities evolve into stationary, solitary-like waves of corresponding polarity, for which we found the amplitude-width, amplitude-velocity, mass-amplitude, and energy-amplitude relations. Small-amplitude impulses to a good approximation can be described by the modified Korteweg-de Vries equation, but larger waves tend to become wide, and absolute value of their amplitude is bounded by the upper limit. Authors thank prof. K.G. Lamb for the opportunity to use the program code for numerical simulations of Euler equations. The research was supported by RFBR (09-05-00447, 09-05-00204) and by President of RF (MD-3024.2008.5 for young doctors of science).

Large Deviations in Weakly Interacting Boundary Driven Lattice Gases

NASA Astrophysics Data System (ADS)

van Wijland, Frédéric; Rácz, Zoltán

2005-01-01

One-dimensional, boundary-driven lattice gases with local interactions are studied in the weakly interacting limit. The density profiles and the correlation functions are calculated to first order in the interaction strength for zero-range and short-range processes differing only in the specifics of the detailed-balance dynamics. Furthermore, the effective free-energy (large-deviation function) and the integrated current distribution are also found to this order. From the former, we find that the boundary drive generates long-range correlations only for the short-range dynamics while the latter provides support to an additivity principle recently proposed by Bodineau and Derrida.

Insights into asthenospheric anisotropy and deformation in Mainland China

NASA Astrophysics Data System (ADS)

Zhu, Tao

2018-03-01

Seismic anisotropy can provide direct constraints on asthenospheric deformation which also can be induced by the inherent mantle flow within our planet. Mantle flow calculations thus have been an effective tool to probe asthenospheric anisotropy. To explore the source of seismic anisotropy, asthenospheric deformation and the effects of mantle flow on seismic anisotropy in Mainland China, mantle flow models driven by plate motion (plate-driven) and by a combination of plate motion and mantle density heterogeneity (plate-density-driven) are used to predict the fast polarization direction of shear wave splitting. Our results indicate that: (1) plate-driven or plate-density-driven mantle flow significantly affects the predicted fast polarization direction when compared with simple asthenospheric flow commonly used in interpreting the asthenospheric source of seismic anisotropy, and thus new insights are presented; (2) plate-driven flow controls the fast polarization direction while thermal mantle flow affects asthenospheric deformation rate and local deformation direction significantly; (3) asthenospheric flow is an assignable contributor to seismic anisotropy, and the asthenosphere is undergoing low, large or moderate shear deformation controlled by the strain model, the flow plane/flow direction model or both in most regions of central and eastern China; and (4) the asthenosphere is under more rapid extension deformation in eastern China than in western China.

Two-fluid flowing equilibria of spherical torus sustained by coaxial helicity injection

NASA Astrophysics Data System (ADS)

Kanki, Takashi; Steinhauer, Loren; Nagata, Masayoshi

2007-11-01

Two-dimensional equilibria in helicity-driven systems using two-fluid model were previously computed, showing the existence of an ultra-low-q spherical torus (ST) configuration with diamagnetism and higher beta. However, this computation assumed purely toroidal ion flow and uniform density. The purpose of the present study is to apply the two-fluid model to the two-dimensional equilibria of helicity-driven ST with non-uniform density and both toroidal and poloidal flows for each species by means of the nearby-fluids procedure, and to explore their properties. We focus our attention on the equilibria relevant to the HIST device, which are characterized by either driven or decaying λ profiles. The equilibrium for the driven λ profile has a diamagnetic toroidal field, high-β (βt = 32%), and centrally broad density. By contrast, the decaying equilibrium has a paramagnetic toroidal field, low-β (βt = 10%), and centrally peaked density with a steep gradient in the outer edge region. In the driven case, the toroidal ion and electron flows are in the same direction, and two-fluid effects are less important since the ExB drift is dominant. In the decaying case, the toroidal ion and electron flows are opposite in the outer edge region, and two-fluid effects are significant locally in the edge due to the ion diamagnetic drift.

TURBULENT TRANSPORT IN A STRONGLY STRATIFIED FORCED SHEAR LAYER WITH THERMAL DIFFUSION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Garaud, Pascale

2016-04-10

This work presents numerical results on the transport of heat and chemical species by shear-induced turbulence in strongly stratified, thermally diffusive environments. The shear instabilities driven in this regime are sometimes called “secular” shear instabilities, and can take place when the Richardson number of the flow is large, provided the Péclet number is small. We have identified a set of simple criteria to determine whether these instabilities can take place or not. Generally speaking, we find that they may be relevant whenever the thermal diffusivity of the fluid is very large (typically larger than 10{sup 14} cm{sup 2} s{sup −1}),more » which is the case in the outer layers of high-mass stars (M ≥ 10 M{sub ⊙}), for instance. Using a simple model setup in which the shear is forced by a spatially sinusoidal, constant-amplitude body-force, we have identified several regimes ranging from effectively unstratified to very strongly stratified, each with its own set of dynamical properties. Unless the system is in one of the two extreme regimes (effectively unstratified or completely stable), however, we find that (1) only about 10% of the input power is used toward heat transport, while the remaining 90% is viscously dissipated; (2) that the effective compositional mixing coefficient is well-approximated by the model of Zahn, with D ≃ 0.02κ{sub T}/J where κ{sub T} is the thermal diffusivity and J is the Richardson number. These results need to be confirmed, however, with simulations in different model setups and at higher effective Reynolds number.« less

Investigation of Stable Atmospheric Stratification Effect on the Dynamics of Descending Vortex Pairs

DOT National Transportation Integrated Search

1979-02-01

The physics of vortex flows in stratified fluids is studied with the objective of determining the influence of stable stratification on the descent of aircraft vortex pairs. Vortex rings descending into linear and discontinuous density stratification...

Quantifying the dynamic density driven convection in high permeability packed beds.

PubMed

Teng, Ying; Jiang, Lanlan; Fan, Yingting; Liu, Yu; Wang, Dayong; Abudula, Abuliti; Song, Yongchen

2017-06-01

The density driven convection phenomenon is expected to have a significant and positive role in CO 2 geological storage capacity and safety. The onset and development of density-driven convective on the core scale is critical to understand the mass transfer mechanism. In this paper, laboratory experiments were conducted to investigate the density-driven convective in a vertical tube. The deuterium oxide (D 2 O)/manganese chloride (MnCl 2 ) water solution in water or brine were as an analog for CO 2 -rich brine in original brine. Experiments are repeated with variations in permeability to vary the characteristic Rayleigh number. Based on the MRI technology, the intensity images showed the interface clearly, reflecting the transition from diffusion to convective. With the echo-multi-slice pulse sequence method, the intensity images can be obtained as 2min 8s. For the denser fluid pairs, fingers appeared, propagated, coalesced and multi-fingers formed. The finger growth rate of the convective was visualized as three distinct periods: rising, stable and declining. Detailed information regarding the wave number, wave length, onset time and mixing time as functions of Rayleigh number are developed. Copyright © 2017 Elsevier Inc. All rights reserved.

Numerical Study of the Buoyancy-Driven Flow in a Four-Electrode Rectangular Electrochemical Cell

NASA Astrophysics Data System (ADS)

Sun, Zhanyu; Agafonov, Vadim; Rice, Catherine; Bindler, Jacob

2009-11-01

Two-dimensional numerical simulation is done on the buoyancy-driven flow in a four-electrode rectangular electrochemical cell. Two kinds of electrode layouts, the anode-cathode-cathode-anode (ACCA) and the cathode-anode-anode-cathode (CAAC) layouts, are studied. In the ACCA layout, the two anodes are placed close to the channel outlets while the two cathodes are located between the two anodes. The CAAC layout can be converted from the ACCA layout by applying higher electric potential on the two middle electrodes. Density gradient was generated by the electrodic reaction I3^-+2e^- =3I^-. When the electrochemical cell is accelerated axially, buoyancy-driven flow occurs. In our model, electro-neutrality is assumed except at the electrodes. The Navier-Stokes equations with the Boussinesq approximation and the Nernst-Planck equations are employed to model the momentum and mass transports, respectively. It is found that under a given axial acceleration, the electrolyte density between the two middle electrodes determines the bulk flow through the electrochemical cell. The cathodic current difference is found to be able to measure the applied acceleration. Other important electro-hydrodynamic characteristics are also discussed.

Current-driven dynamics of skyrmions stabilized in MnSi nanowires revealed by topological Hall effect

PubMed Central

Liang, Dong; DeGrave, John P.; Stolt, Matthew J.; Tokura, Yoshinori; Jin, Song

2015-01-01

Skyrmions hold promise for next-generation magnetic storage as their nanoscale dimensions may enable high information storage density and their low threshold for current-driven motion may enable ultra-low energy consumption. Skyrmion-hosting nanowires not only serve as a natural platform for magnetic racetrack memory devices but also stabilize skyrmions. Here we use the topological Hall effect (THE) to study phase stability and current-driven dynamics of skyrmions in MnSi nanowires. THE is observed in an extended magnetic field-temperature window (15–30 K), suggesting stabilization of skyrmions in nanowires compared with the bulk. Furthermore, we show in nanowires that under the high current density of 108–109 A m−2, the THE decreases with increasing current densities, which demonstrates the current-driven motion of skyrmions generating the emergent electric field in the extended skyrmion phase region. These results open up the exploration of skyrmions in nanowires for fundamental physics and magnetic storage technologies. PMID:26400204

Development of Uniform Microstructures in Immiscible Alloys by Processing in a Low-Gravity Environment

NASA Technical Reports Server (NTRS)

Grugel, R. N.; Brush, L. N.

1996-01-01

Highly segregated macrostructures tend to develop during processing of hypermonotectic alloys because of the density difference existing between the two liquid phases. The approximately 4.6 seconds of low-gravity provided by Marshall Space Flight Center's 105 meter drop tube was utilized to minimize density-driven separation and promote uniform microstructures in hypermonotectic Ag-Ni and Ag-Mn alloys. For the Ag-Ni alloys a numerical model was developed to track heat flow and solidification of the bi-metal drop configuration. Results, potential applications, and future work are presented.

Computed tomography of adult colonic intussusception: clinical and experimental studies

DOE Office of Scientific and Technical Information (OSTI.GOV)

Iko, B.O.; Teal, J.S.; Siram, S.M.

1984-10-01

The CT features of a case of adult ileocolic intussusception and of experimentally induced ileocolic, cecocolic, and colocolic intussusceptions are presented. Both the clinical and experimental cases demonstrated (1) target masses with enveloped, eccentrically located areas of low density and (2) interspersed low- and high-density stripes within the intussusception producing a layered or stratified pattern. This layered pattern of abdominal masses may be characteristic of intussusceptions regardless of location.

Influence of lifestyle factors on mammographic density in postmenopausal women.

PubMed

Brand, Judith S; Czene, Kamila; Eriksson, Louise; Trinh, Thang; Bhoo-Pathy, Nirmala; Hall, Per; Celebioglu, Fuat

2013-01-01

Mammographic density is a strong risk factor for breast cancer. Apart from hormone replacement therapy (HRT), little is known about lifestyle factors that influence breast density. We examined the effect of smoking, alcohol and physical activity on mammographic density in a population-based sample of postmenopausal women without breast cancer. Lifestyle factors were assessed by a questionnaire and percentage and area measures of mammographic density were measured using computer-assisted software. General linear models were used to assess the association between lifestyle factors and mammographic density and effect modification by body mass index (BMI) and HRT was studied. Overall, alcohol intake was positively associated with percent mammographic density (P trend  = 0.07). This association was modified by HRT use (P interaction  = 0.06): increasing alcohol intake was associated with increasing percent density in current HRT users (P trend  = 0.01) but not in non-current users (P trend  = 0.82). A similar interaction between alcohol and HRT was found for the absolute dense area, with a positive association being present in current HRT users only (P interaction  = 0.04). No differences in mammographic density were observed across categories of smoking and physical activity, neither overall nor in stratified analyses by BMI and HRT use. Increasing alcohol intake is associated with an increase in mammography density, whereas smoking and physical activity do not seem to influence density. The observed interaction between alcohol and HRT may pose an opportunity for HRT users to lower their mammographic density and breast cancer risk.

Influence of Lifestyle Factors on Mammographic Density in Postmenopausal Women

PubMed Central

Brand, Judith S.; Czene, Kamila; Eriksson, Louise; Trinh, Thang; Bhoo-Pathy, Nirmala; Hall, Per; Celebioglu, Fuat

2013-01-01

Background Mammographic density is a strong risk factor for breast cancer. Apart from hormone replacement therapy (HRT), little is known about lifestyle factors that influence breast density. Methods We examined the effect of smoking, alcohol and physical activity on mammographic density in a population-based sample of postmenopausal women without breast cancer. Lifestyle factors were assessed by a questionnaire and percentage and area measures of mammographic density were measured using computer-assisted software. General linear models were used to assess the association between lifestyle factors and mammographic density and effect modification by body mass index (BMI) and HRT was studied. Results Overall, alcohol intake was positively associated with percent mammographic density (P trend  = 0.07). This association was modified by HRT use (P interaction  = 0.06): increasing alcohol intake was associated with increasing percent density in current HRT users (P trend  = 0.01) but not in non-current users (P trend  = 0.82). A similar interaction between alcohol and HRT was found for the absolute dense area, with a positive association being present in current HRT users only (P interaction  = 0.04). No differences in mammographic density were observed across categories of smoking and physical activity, neither overall nor in stratified analyses by BMI and HRT use. Conclusions Increasing alcohol intake is associated with an increase in mammography density, whereas smoking and physical activity do not seem to influence density. The observed interaction between alcohol and HRT may pose an opportunity for HRT users to lower their mammographic density and breast cancer risk. PMID:24349146

Dense blocks of energetic ions driven by multi-petawatt lasers

PubMed Central

Weng, S. M.; Liu, M.; Sheng, Z. M.; Murakami, M.; Chen, M.; Yu, L. L.; Zhang, J.

2016-01-01

Laser-driven ion accelerators have the advantages of compact size, high density, and short bunch duration over conventional accelerators. Nevertheless, it is still challenging to simultaneously enhance the yield and quality of laser-driven ion beams for practical applications. Here we propose a scheme to address this challenge via the use of emerging multi-petawatt lasers and a density-modulated target. The density-modulated target permits its ions to be uniformly accelerated as a dense block by laser radiation pressure. In addition, the beam quality of the accelerated ions is remarkably improved by embedding the target in a thick enough substrate, which suppresses hot electron refluxing and thus alleviates plasma heating. Particle-in-cell simulations demonstrate that almost all ions in a solid-density plasma of a few microns can be uniformly accelerated to about 25% of the speed of light by a laser pulse at an intensity around 1022 W/cm2. The resulting dense block of energetic ions may drive fusion ignition and more generally create matter with unprecedented high energy density. PMID:26924793

Nonlinear modeling of wave-topography interactions, shear instabilities and shear induced wave breaking using vortex method

NASA Astrophysics Data System (ADS)

Guha, Anirban

2017-11-01

Theoretical studies on linear shear instabilities as well as different kinds of wave interactions often use simple velocity and/or density profiles (e.g. constant, piecewise) for obtaining good qualitative and quantitative predictions of the initial disturbances. Moreover, such simple profiles provide a minimal model to obtain a mechanistic understanding of shear instabilities. Here we have extended this minimal paradigm into nonlinear domain using vortex method. Making use of unsteady Bernoulli's equation in presence of linear shear, and extending Birkhoff-Rott equation to multiple interfaces, we have numerically simulated the interaction between multiple fully nonlinear waves. This methodology is quite general, and has allowed us to simulate diverse problems that can be essentially reduced to the minimal system with interacting waves, e.g. spilling and plunging breakers, stratified shear instabilities (Holmboe, Taylor-Caulfield, stratified Rayleigh), jet flows, and even wave-topography interaction problem like Bragg resonance. We found that the minimal models capture key nonlinear features (e.g. wave breaking features like cusp formation and roll-ups) which are observed in experiments and/or extensive simulations with smooth, realistic profiles.

Homogeneous purely buoyancy driven turbulent flow

NASA Astrophysics Data System (ADS)

Arakeri, Jaywant; Cholemari, Murali; Pawar, Shashikant

2010-11-01

An unstable density difference across a long vertical tube open at both ends leads to convection that is axially homogeneous with a linear density gradient. We report results from such tube convection experiments, with driving density caused by salt concentration difference or temperature difference. At high enough Rayleigh numbers (Ra) the convection is turbulent with zero mean flow and zero mean Reynolds shear stresses; thus turbulent production is purely by buoyancy. We observe different regimes of turbulent convection. At very high Ra the Nusselt number scales as the square root of the Rayleigh number, giving the so-called "ultimate regime" of convection predicted for Rayleigh-Benard convection in limit of infinite Ra. Turbulent convection at intermediate Ra, the Nusselt number scales as Ra^0.3. In both regimes, the flux and the Taylor scale Reynolds number are more than order of magnitude larger than those obtained in Rayleigh-Benard convection. Absence of a mean flow makes this an ideal flow to study shear free turbulence near a wall.

Simulation and study of stratified flows around finite bodies

NASA Astrophysics Data System (ADS)

Gushchin, V. A.; Matyushin, P. V.

2016-06-01

The flows past a sphere and a square cylinder of diameter d moving horizontally at the velocity U in a linearly density-stratified viscous incompressible fluid are studied. The flows are described by the Navier-Stokes equations in the Boussinesq approximation. Variations in the spatial vortex structure of the flows are analyzed in detail in a wide range of dimensionless parameters (such as the Reynolds number Re = Ud/ ν and the internal Froude number Fr = U/( Nd), where ν is the kinematic viscosity and N is the buoyancy frequency) by applying mathematical simulation (on supercomputers of Joint Supercomputer Center of the Russian Academy of Sciences) and three-dimensional flow visualization. At 0.005 < Fr < 100, the classification of flow regimes for the sphere (for 1 < Re < 500) and for the cylinder (for 1 < Re < 200) is improved. At Fr = 0 (i.e., at U = 0), the problem of diffusion-induced flow past a sphere leading to the formation of horizontal density layers near the sphere's upper and lower poles is considered. At Fr = 0.1 and Re = 50, the formation of a steady flow past a square cylinder with wavy hanging density layers in the wake is studied in detail.

Oscillatory interfacial instability between miscible fluids

NASA Astrophysics Data System (ADS)

Shevtsova, Valentina; Gaponenko, Yuri; Mialdun, Aliaksandr; Torregrosa, Marita; Yasnou, Viktar

Interfacial instabilities occurring between two fluids are of fundamental interest in fluid dynamics, biological systems and engineering applications such as liquid storage, solvent extraction, oil recovery and mixing. Horizontal vibrations applied to stratified layers of immiscible liquids may generate spatially periodic waving of the interface, stationary in the reference frame of the vibrated cell, referred to as a "frozen wave". We present experimental evidence that frozen wave instability exists between two ordinary miscible liquids of similar densities and viscosities. At the experiments and at the numerical model, two superimposed layers of ordinary liquids, water-alcohol of different concentrations, are placed in a closed cavity in a gravitationally stable configuration. The density and viscosity of these fluids are somewhat similar. Similar to the immiscible fluids this instability has a threshold. When the value of forcing is increased the amplitudes of perturbations grow continuously displaying a saw-tooth structure. The decrease of gravity drastically changes the structure of frozen waves.

Entropy-driven phase transitions of entanglement

NASA Astrophysics Data System (ADS)

Facchi, Paolo; Florio, Giuseppe; Parisi, Giorgio; Pascazio, Saverio; Yuasa, Kazuya

2013-05-01

We study the behavior of bipartite entanglement at fixed von Neumann entropy. We look at the distribution of the entanglement spectrum, that is, the eigenvalues of the reduced density matrix of a quantum system in a pure state. We report the presence of two continuous phase transitions, characterized by different entanglement spectra, which are deformations of classical eigenvalue distributions.

Comparing the statistics of interstellar turbulence in simulations and observations. Solenoidal versus compressive turbulence forcing

NASA Astrophysics Data System (ADS)

Federrath, C.; Roman-Duval, J.; Klessen, R. S.; Schmidt, W.; Mac Low, M.-M.

2010-03-01

Context. Density and velocity fluctuations on virtually all scales observed with modern telescopes show that molecular clouds (MCs) are turbulent. The forcing and structural characteristics of this turbulence are, however, still poorly understood. Aims: To shed light on this subject, we study two limiting cases of turbulence forcing in numerical experiments: solenoidal (divergence-free) forcing and compressive (curl-free) forcing, and compare our results to observations. Methods: We solve the equations of hydrodynamics on grids with up to 10243 cells for purely solenoidal and purely compressive forcing. Eleven lower-resolution models with different forcing mixtures are also analysed. Results: Using Fourier spectra and Δ-variance, we find velocity dispersion-size relations consistent with observations and independent numerical simulations, irrespective of the type of forcing. However, compressive forcing yields stronger compression at the same rms Mach number than solenoidal forcing, resulting in a three times larger standard deviation of volumetric and column density probability distributions (PDFs). We compare our results to different characterisations of several observed regions, and find evidence of different forcing functions. Column density PDFs in the Perseus MC suggest the presence of a mainly compressive forcing agent within a shell, driven by a massive star. Although the PDFs are close to log-normal, they have non-Gaussian skewness and kurtosis caused by intermittency. Centroid velocity increments measured in the Polaris Flare on intermediate scales agree with solenoidal forcing on that scale. However, Δ-variance analysis of the column density in the Polaris Flare suggests that turbulence is driven on large scales, with a significant compressive component on the forcing scale. This indicates that, although likely driven with mostly compressive modes on large scales, turbulence can behave like solenoidal turbulence on smaller scales. Principal component analysis of G216-2.5 and most of the Rosette MC agree with solenoidal forcing, but the interior of an ionised shell within the Rosette MC displays clear signatures of compressive forcing. Conclusions: The strong dependence of the density PDF on the type of forcing must be taken into account in any theory using the PDF to predict properties of star formation. We supply a quantitative description of this dependence. We find that different observed regions show evidence of different mixtures of compressive and solenoidal forcing, with more compressive forcing occurring primarily in swept-up shells. Finally, we emphasise the role of the sonic scale for protostellar core formation, because core formation close to the sonic scale would naturally explain the observed subsonic velocity dispersions of protostellar cores. A movie is only available in electronic form at http://www.aanda.org

Genetic effects on bone mass and turnover-relevance to black/white differences.

PubMed

Parfitt, A M

1997-08-01

The mass of a bone is given by its volume and its apparent density--mass per unit external volume. Most measurements of so-called density are of mass incompletely normalized by some index of bone size. Genes control about 60% to 75% of the variance of peak bone mass/density and a much smaller proportion of the variance in rate of loss. Genetic influence on bone mass/density are mediated in large part by body size, bone size, and muscle mass. Most of the fifty-fold increase in bone mass from birth to maturity is due to bone growth, which is linked to muscle growth and bodily growth. Three-D apparent bone density in the vertebrae increases about 15% during the pubertal growth spurt. The genetic potential for bone accumulation can be frustrated by insufficient calcium intake, disruption of the calendar of puberty and inadequate physical activity. The growing skeleton is much more responsive than the mature skeleton to the osteotrophic effect of exercise, which is mediated by the detection of deviations from a target value for strain, and orchestration of cellular responses that restore the target value, processes collectively termed the mechanostat. Production of metaphyseal cancellous bone and growth in length are both linked to endochondral ossification, which is driven by growth plate cartilage cell proliferation. Production of diaphyseal cortical bone and growth in width are both linked to periosteal apposition, which is driven by osteoblast precursor proliferation. During adolescence trabeculae and cortices become thicker by net endosteal apposition, which increases apparent density. Two lines of evidence support a genetic basis for black/white differences in bone mass. First, the magnitude (10% to 40%) is incommensurate with known nongenetic factors. Second, the difference is already evident in the fetus and increases progressively during growth, especially in adolescence; the difference in peak bone mass persists throughout life. The genetic determination of bone mass is mediated by two classes of gene. The first regulates growth of the body, including muscles and bones, under the control of a master gene or set of genes whose products function as the sizostat. The second regulates the increase in apparent bone density in response to load bearing, under the control of a master gene or set of genes whose products function as the mechanostat.

Flow of two immiscible fluids in a periodically constricted tube: Transitions to stratified, segmented, churn, spray, or segregated flow

NASA Astrophysics Data System (ADS)

Fraggedakis, D.; Kouris, Ch.; Dimakopoulos, Y.; Tsamopoulos, J.

2015-08-01

We study the flow of two immiscible, Newtonian fluids in a periodically constricted tube driven by a constant pressure gradient. Our volume-of-fluid algorithm is used to solve the governing equations. First, the code is validated by comparing its predictions to previously reported results for stratified and pulsing flow. Then, it is used to capture accurately all the significant topological changes that take place. Initially, the fluids have a core-annular arrangement, which is found to either remain the same or change to a different arrangement depending on the fluid properties, the pressure driving the flow, or the flow geometry. The flow-patterns that appear are the core-annular, segmented, churn, spray, and segregated flow. The predicted scalings near pinching of the core fluid concur with similarity predictions and earlier numerical results [I. Cohen et al., "Two fluid drop snap-off problem: Experiments and theory," Phys. Rev. Lett. 83, 1147-1150 (1999)]. Flow-pattern maps are constructed in terms of the Reynolds and Weber numbers. Our result provides deeper insights into the mechanism of the pattern transitions and is in agreement with previous studies on core-annular flow [Ch. Kouris and J. Tsamopoulos, "Core-annular flow in a periodically constricted circular tube, I. Steady state, linear stability and energy analysis," J. Fluid Mech. 432, 31-68 (2001) and Ch. Kouris et al., "Comparison of spectral and finite element methods applied to the study of interfacial instabilities of the core-annular flow in an undulating tube," Int. J. Numer. Methods Fluids 39(1), 41-73 (2002)], segmented flow [E. Lac and J. D. Sherwood, "Motion of a drop along the centreline of a capillary in a pressure-driven flow," J. Fluid Mech. 640, 27-54 (2009)], and churn flow [R. Y. Bai et al., "Lubricated pipelining—Stability of core annular-flow. 5. Experiments and comparison with theory," J. Fluid Mech. 240, 97-132 (1992)].

Studies of Current Circulation at Ocean Waste Disposal Sites

NASA Technical Reports Server (NTRS)

Klemas, V. (Principal Investigator); Davis, G.; Henry, R.

1976-01-01

The author has identified the following significant results. Acid waste plume was observed in LANDSAT imagery fourteen times ranging from during dump up to 54 hours after dump. Circulation processes at the waste disposal site are highly storm-dominated, with the majority of the water transport occurring during strong northeasterlies. There is a mean flow to the south along shore. This appears to be due to the fact that northeasterly winds produce stronger currents than those driven by southeasterly winds and by the thermohaline circulation. During the warm months (May through October), the ocean at the dump site stratifies with a distinct thermocline observed during all summer cruising at depths ranging from 10 to 21 m. During stratified conditions, the near-bottom currents were small. Surface currents responded to wind conditions resulting in rapid movement of surface drogues on windy days. Mid-depth drogues showed an intermediate behavior, moving more rapidly as wind velocities increased.

The energetics of heterogeneous deformation in open-cell elastic foams

NASA Astrophysics Data System (ADS)

Gioia, Gustavo; Cuitino, Alberto

2002-03-01

We study the energetics of a model of elastic foams to show that the stretch heterogeneity observed in experiments stems from the lack of convexity of the governing energy functional. The predicted stretch distributions correspond to stratified mixtures of two configurational phases of the foam. Stretching occurs in the form of a phase transition, by growth of one of the phases at the expense of the other. We also compare the predicted mechanical response with experimental data for foams of different densities. Lastly, we perform displacement field measurements using the digital image correlation technique, and find the results to be in agreement with our predictions.

Retrogressive failures recorded in mass transport deposits in the Ursa Basin, Northern Gulf of Mexico

NASA Astrophysics Data System (ADS)

Sawyer, Derek E.; Flemings, Peter B.; Dugan, Brandon; Germaine, John T.

2009-10-01

Clay-rich mass transport deposits (MTDs) in the Ursa Basin, Gulf of Mexico, record failures that mobilized along extensional failure planes and transformed into long runout flows. Failure proceeded retrogressively: scarp formation unloaded adjacent sediment causing extensional failure that drove successive scarp formation updip. This model is developed from three-dimensional seismic reflection data, core and log data from Integrated Ocean Drilling Project (IODP) Expedition 308, and triaxial shear experiments. MTDs are imaged seismically as low-amplitude zones above continuous, grooved, high-amplitude basal reflections and are characterized by two seismic facies. A Chaotic facies typifies the downdip interior, and a Discontinuous Stratified facies typifies the headwalls/sidewalls. The Chaotic facies contains discontinuous, high-amplitude reflections that correspond to flow-like features in amplitude maps: it has higher bulk density, resistivity, and shear strength, than bounding sediment. In contrast, the Discontinuous Stratified facies contains relatively dim reflections that abut against intact pinnacles of parallel-stratified reflections: it has only slightly higher bulk density, resistivity, and shear strength than bounding sediment, and deformation is limited. In both facies, densification is greatest at the base, resulting in a strong basal reflection. Undrained shear tests document strain weakening (sensitivity = 3). We estimate that failure at 30 meters below seafloor will occur when overpressure = 70% of the hydrostatic effective stress: under these conditions soil will liquefy and result in long runout flows.

Stratified Shear Flows In Pipe Geometries

NASA Astrophysics Data System (ADS)

Harabin, George; Camassa, Roberto; McLaughlin, Richard; UNC Joint Fluids Lab Team Team

2015-11-01

Exact and series solutions to the full Navier-Stokes equations coupled to the advection diffusion equation are investigated in tilted three-dimensional pipe geometries. Analytic techniques for studying the three-dimensional problem provide a means for tackling interesting questions such as the optimal domain for mass transport, and provide new avenues for experimental investigation of diffusion driven flows. Both static and time dependent solutions will be discussed. NSF RTG DMS-0943851, NSF RTG ARC-1025523, NSF DMS-1009750.

VALIDITY OF A TWO-DIMENSIONAL MODEL FOR VARIABLE-DENSITY HYDRODYNAMIC CIRCULATION

EPA Science Inventory

A three-dimensional model of temperatures and currents has been formulated to assist in the analysis and interpretation of the dynamics of stratified lakes. In this model, nonlinear eddy coefficients for viscosity and conductivities are included. A two-dimensional model (one vert...

Parametric instability and wave turbulence driven by tidal excitation of internal waves

NASA Astrophysics Data System (ADS)

Le Reun, Thomas; Favier, Benjamin; Le Bars, Michael

2018-04-01

We investigate the stability of stratified fluid layers undergoing homogeneous and periodic tidal deformation. We first introduce a local model which allows to study velocity and buoyancy fluctuations in a Lagrangian domain periodically stretched and sheared by the tidal base flow. While keeping the key physical ingredients only, such a model is efficient to simulate planetary regimes where tidal amplitudes and dissipation are small. With this model, we prove that tidal flows are able to drive parametric subharmonic resonances of internal waves, in a way reminiscent of the elliptical instability in rotating fluids. The growth rates computed via Direct Numerical Simulations (DNS) are in very good agreement with WKB analysis and Floquet theory. We also investigate the turbulence driven by this instability mechanism. With spatio-temporal analysis, we show that it is a weak internal wave turbulence occurring at small Froude and buoyancy Reynolds numbers. When the gap between the excitation and the Brunt-V\\"ais\\"al\\"a frequencies is increased, the frequency spectrum of this wave turbulence displays a -2 power law reminiscent of the high-frequency branch of the Garett and Munk spectrum (Garrett & Munk 1979) which has been measured in the oceans. In addition, we find that the mixing efficiency is altered compared to what is computed in the context of DNS of stratified turbulence excited at small Froude and large buoyancy Reynolds numbers and is consistent with a superposition of waves.

Measurements of fluid transport by controllable vertical migrations of plankton

NASA Astrophysics Data System (ADS)

Houghton, Isabel A.; Dabiri, John O.

2016-11-01

Diel vertical migration of zooplankton has been proposed to be a significant contributor to local and possibly large-scale fluid transport in the ocean. However, studies of this problem to date have been limited to order-of-magnitude estimates based on first principles and a small number of field observations. In this work, we leverage the phototactic behavior of zooplankton to stimulate controllable vertical migrations in the laboratory and to study the associated fluid transport and mixing. Building upon a previous prototype system, a laser guidance system induces vertical swimming of brine shrimp (Artemia salina) in a 2.1 meter tall, density-stratified water tank. The animal swimming speed and spacing during the controlled vertical migration is characterized with video analysis. A schlieren imaging system is utilized to visualize density perturbations to a stable stratification for quantification of fluid displacement length scales and restratification timescales. These experiments can add to our understanding of the dynamics of active particles in stratified flows. NSF and US-Israel Binational Science Foundation.

Thermal stratification in LH2 tank of cryogenic propulsion stage tested in ISRO facility

NASA Astrophysics Data System (ADS)

Xavier, M.; Raj, R. Edwin; Narayanan, V.

2017-02-01

Liquid oxygen and hydrogen are used as oxidizer and fuel respectively in cryogenic propulsion system. These liquids are stored in foam insulated tanks of cryogenic propulsion system and are pressurized using warm pressurant gas supplied for tank pressure maintenance during cryogenic engine operation. Heat leak to cryogenic propellant tank causes buoyancy driven liquid stratification resulting in formation of warm liquid stratum at liquid free surface. This warm stratum is further heated by the admission of warm pressurant gas for tank pressurization during engine operation. Since stratified layer temperature has direct bearing on the cavitation free operation of turbo pumps integrated in cryogenic engine, it is necessary to model the thermal stratification for predicting stratified layer temperature and mass of stratified liquid in tank at the end of engine operation. These inputs are required for estimating the minimum pressure to be maintained by tank pressurization system. This paper describes configuration of cryogenic stage for ground qualification test, stage hot test sequence, a thermal model and its results for a foam insulated LH2 tank subjected to heat leak and pressurization with hydrogen gas at 200 K during liquid outflow at 38 lps for engine operation. The above model considers buoyancy flow in free convection boundary layer caused by heat flux from tank wall and energy transfer from warm pressurant gas etc. to predict temperature of liquid stratum and mass of stratified liquid in tank at the end of engine operation in stage qualification tests carried out in ISRO facility.

CFD Analyses of Air-Ingress Accident for VHTRs

NASA Astrophysics Data System (ADS)

Ham, Tae Kyu

The Very High Temperature Reactor (VHTR) is one of six proposed Generation-IV concepts for the next generation of nuclear powered plants. The VHTR is advantageous because it is able to operate at very high temperatures, thus producing highly efficient electrical generation and hydrogen production. A critical safety event of the VHTR is a loss-of-coolant accident. This accident is initiated, in its worst-case scenario, by a double-ended guillotine break of the cross vessel that connects the reactor vessel and the power conversion unit. Following the depressurization process, the air (i.e., the air and helium mixture) in the reactor cavity could enter the reactor core causing an air-ingress event. In the event of air-ingress into the reactor core, the high-temperature in-core graphite structures will chemically react with the air and could lose their structural integrity. We designed a 1/8th scaled-down test facility to develop an experimental database for studying the mechanisms involved in the air-ingress phenomenon. The current research focuses on the analysis of the air-ingress phenomenon using the computational fluid dynamics (CFD) tool ANSYS FLUENT for better understanding of the air-ingress phenomenon. The anticipated key steps in the air-ingress scenario for guillotine break of VHTR cross vessel are: 1) depressurization; 2) density-driven stratified flow; 3) local hot plenum natural circulation; 4) diffusion into the reactor core; and 5) global natural circulation. However, the OSU air-ingress test facility covers the time from depressurization to local hot plenum natural circulation. Prior to beginning the CFD simulations for the OSU air-ingress test facility, benchmark studies for the mechanisms which are related to the air-ingress accident, were performed to decide the appropriate physical models for the accident analysis. In addition, preliminary experiments were performed with a simplified 1/30th scaled down acrylic set-up to understand the air-ingress mechanism and to utilize the CFD simulation in the analysis of the phenomenon. Previous air-ingress studies simulated the depressurization process using simple assumptions or 1-D system code results. However, recent studies found flow oscillations near the end of the depressurization which could influence the next stage of the air-ingress accident. Therefore, CFD simulations were performed to examine the air-ingress mechanisms from the depressurization through the establishment of local natural circulation initiate. In addition to the double-guillotine break scenario, there are other scenarios that can lead to an air-ingress event such as a partial break were in the cross vessel with various break locations, orientations, and shapes. These additional situations were also investigated. The simulation results for the OSU test facility showed that the discharged helium coolant from a reactor vessel during the depressurization process will be mixed with the air in the containment. This process makes the density of the gas mixture in the containment lower and the density-driven air-ingress flow slower because the density-driven flow is established by the density difference of the gas species between the reactor vessel and the containment. In addition, for the simulations with various initial and boundary conditions, the simulation results showed that the total accumulated air in the containment collapsed within 10% standard deviation by: 1. multiplying the density ratio and viscosity ratio of the gas species between the containment and the reactor vessel and 2. multiplying the ratio of the air mole fraction and gas temperature to the reference value. By replacing the gas mixture in the reactor cavity with a gas heavier than the air, the air-ingress speed slowed down. Based on the understanding of the air-ingress phenomena for the GT-MHR air-ingress scenario, several mitigation measures of air-ingress accident are proposed. The CFD results are utilized to plan experimental strategy and apparatus installation to obtain the best results when conducting an experiment. The validation of the generated CFD solutions will be performed with the OSU air-ingress experimental results. (Abstract shortened by UMI.).

Large eddy simulations of time-dependent and buoyancy-driven channel flows

NASA Technical Reports Server (NTRS)

Cabot, William H.

1993-01-01

The primary goal of this work has been to assess the performance of the dynamic SGS model in the large eddy simulation (LES) of channel flows in a variety of situations, viz., in temporal development of channel flow turned by a transverse pressure gradient and especially in buoyancy-driven turbulent flows such as Rayleigh-Benard and internally heated channel convection. For buoyancy-driven flows, there are additional buoyant terms that are possible in the base models, and one objective has been to determine if the dynamic SGS model results are sensitive to such terms. The ultimate goal is to determine the minimal base model needed in the dynamic SGS model to provide accurate results in flows with more complicated physical features. In addition, a program of direct numerical simulation (DNS) of fully compressible channel convection has been undertaken to determine stratification and compressibility effects. These simulations are intended to provide a comparative base for performing the LES of compressible (or highly stratified, pseudo-compressible) convection at high Reynolds number in the future.

Alcohol Prevention on College Campuses: The Moderating Effect of the Alcohol Environment on the Effectiveness of Social Norms Marketing Campaigns*

PubMed Central

Scribner, Richard A.; Theall, Katherine P.; Mason, Karen; Simonsen, Neal; Schneider, Shari Kessel; Towvim, Laura Gomberg; DeJong, William

2011-01-01

Objective: Evaluations of social norms marketing campaigns to reduce college student drinking have produced conflicting results. This study examines whether the effectiveness of such campaigns may be moderated by on-premise alcohol outlet density in the surrounding community. Method: Multilevel analyses were conducted of student survey responses (N = 19,838) from 32 U.S. colleges that took part in one of two 4-year randomized, controlled trials completed for the Social Norms Marketing Research Project (SNMRP). In the models, students by year were nested within treatment (n = 16) and control group (n = 16) campuses, which were characterized by the on-premise outlet density in their surrounding community. The moderating effect of outlet density was introduced into the models as an interaction between the treatment effect (i.e., the effect of the social norms marketing campaigns over time) and outlet density. The models were also stratified by campus alcohol outlet density (high vs. low) to examine the effect of the intervention in each type of setting. Results: There was a significant interaction between the treatment effect and on-premise alcohol outlet density for one of the drinking outcomes targeted by the SNMRP intervention, the number of drinks when partying, and marginal evidence of interaction effects for two other outcomes, maximum recent consumption and a composite drinking scale. In stratified analyses, an intervention effect was observed for three of the four outcomes among students from campuses with lower on-premise alcohol outlet density, whereas no intervention effect was observed among students from campuses with higher on-premise alcohol outlet density. Conclusions: The findings suggest that the campus alcohol environment moderates the effect of social norms marketing interventions. Social norms marketing intervention may be less effective on campuses with higher densities of on-sale alcohol outlets. PMID:21388596

Geo-spatial aspects of acceptance of illegal hunting of large carnivores in Scandinavia.

PubMed

Gangaas, Kristin E; Kaltenborn, Bjørn P; Andreassen, Harry P

2013-01-01

Human-carnivore conflicts are complex and are influenced by: the spatial distribution of the conflict species; the organisation and intensity of management measures such as zoning; historical experience with wildlife; land use patterns; and local cultural traditions. We have used a geographically stratified sampling of social values and attitudes to provide a novel perspective to the human - wildlife conflict. We have focused on acceptance by and disagreements between residents (measured as Potential Conflict Index; PCI) towards illegal hunting of four species of large carnivores (bear, lynx, wolf, wolverine). The study is based on surveys of residents in every municipality in Sweden and Norway who were asked their opinion on illegal hunting. Our results show how certain social values are associated with acceptance of poaching, and how these values differ geographically independent of carnivore abundance. Our approach differs from traditional survey designs, which are often biased towards urban areas. Although these traditional designs intend to be representative of a region (i.e. a random sample from a country), they tend to receive relatively few respondents from rural areas that experience the majority of conflict with carnivores. Acceptance of poaching differed significantly between Norway (12.7-15.7% of respondents) and Sweden (3.3-4.1% of respondents). We found the highest acceptance of illegal hunting in rural areas with free-ranging sheep and strong hunting traditions. Disagreements between residents (as measured by PCI) were highest in areas with intermediate population density. There was no correlation between carnivore density and either acceptance of illegal hunting or PCI. A strong positive correlation between acceptance of illegal hunting and PCI showed that areas with high acceptance of illegal hunting are areas with high potential conflict between people. Our results show that spatially-stratified surveys are required to reveal the large scale patterns in social dynamics of human-wildlife conflicts.

Mixing driven by transient buoyancy flows. I. Kinematics

NASA Astrophysics Data System (ADS)

Duval, W. M. B.; Zhong, H.; Batur, C.

2018-05-01

Mixing of two miscible liquids juxtaposed inside a cavity initially separated by a divider, whose buoyancy-driven motion is initiated via impulsive perturbation of divider motion that can generate the Richtmyer-Meshkov instability, is investigated experimentally. The measured Lagrangian history of interface motion that contains the continuum mechanics of mixing shows self-similar nearly Gaussian length stretch distribution for a wide range of control parameters encompassing an approximate Hele-Shaw cell to a three-dimensional cavity. Because of the initial configuration of the interface which is parallel to the gravitational field, we show that at critical initial potential energy mixing occurs through the stretching of the interface, which shows frontogenesis, and folding, owing to an overturning motion that results in unstable density stratification and produces an ideal condition for the growth of the single wavelength Rayleigh-Taylor instability. The initial perturbation of the interface and flow field generates the Kelvin-Helmholtz instability and causes kinks at the interface, which grow into deep fingers during overturning motion and unfold into local whorl structures that merge and self-organize into the Rayleigh-Taylor morphology (RTM) structure. For a range of parametric space that yields two-dimensional flows, the unfolding of the instability through a supercritical bifurcation yields an asymmetric pairwise structure exhibiting smooth RTM that transitions to RTM fronts with fractal structures that contain small length scales for increasing Peclet numbers. The late stage of the RTM structure unfolds into an internal breakwave that breaks down through wall and internal collision and sets up the condition for self-induced sloshing that decays exponentially as the two fluids become stably stratified with a diffusive region indicating local molecular diffusion.

Magnetically Driven Accretion Disk Winds and Ultra-fast Outflows in PG 1211+143

NASA Astrophysics Data System (ADS)

Fukumura, Keigo; Tombesi, Francesco; Kazanas, Demosthenes; Shrader, Chris; Behar, Ehud; Contopoulos, Ioannis

2015-05-01

We present a study of X-ray ionization of MHD accretion-disk winds in an effort to constrain the physics underlying the highly ionized ultra-fast outflows (UFOs) inferred by X-ray absorbers often detected in various sub classes of Seyfert active galactic nuclei (AGNs). Our primary focus is to show that magnetically driven outflows are indeed physically plausible candidates for the observed outflows accounting for the AGN absorption properties of the present X-ray spectroscopic observations. Employing a stratified MHD wind launched across the entire AGN accretion disk, we calculate its X-ray ionization and the ensuing X-ray absorption-line spectra. Assuming an appropriate ionizing AGN spectrum, we apply our MHD winds to model the absorption features in an XMM-Newton/EPIC spectrum of the narrow-line Seyfert, PG 1211+143. We find, through identifying the detected features with Fe Kα transitions, that the absorber has a characteristic ionization parameter of log (ξc[erg cm s-1]) ≃ 5-6 and a column density on the order of NH ≃ 1023 cm-2 outflowing at a characteristic velocity of vc/c ≃ 0.1-0.2 (where c is the speed of light). The best-fit model favors its radial location at rc ≃ 200 Ro (Ro is the black hole’s innermost stable circular orbit), with an inner wind truncation radius at Rt ≃ 30 Ro. The overall K-shell feature in the data is suggested to be dominated by Fe xxv with very little contribution from Fe xxvi and weakly ionized iron, which is in good agreement with a series of earlier analyses of the UFOs in various AGNs, including PG 1211+143.

Ion acceleration in electrostatic collisionless shock: on the optimal density profile for quasi-monoenergetic beams

DOE Office of Scientific and Technical Information (OSTI.GOV)

Boella, E.; Fiúza, F.; Novo, A. Stockem

Here, a numerical study on ion acceleration in electrostatic shock waves is presented, with the aim of determining the best plasma configuration to achieve quasi-monoenergetic ion beams in laser-driven systems. It was recently shown that tailored near-critical density plasmas characterized by a long-scale decreasing rear density profile lead to beams with low energy spread (Fiúza et al 2012 Phys. Rev. Lett. 109 215001). In this work, a detailed parameter scan investigating different plasma scale lengths is carried out. As result, the optimal plasma spatial scale length that allows for minimizing the energy spread while ensuring a significant reflection of ionsmore » by the shock is identified. Furthermore, a new configuration where the required profile has been obtained by coupling micro layers of different densities is proposed. Lastly, results show that this new engineered approach is a valid alternative, guaranteeing a low energy spread with a higher level of controllability.« less

Structure and mechanism of diet specialisation: testing models of individual variation in resource use with sea otters

USGS Publications Warehouse

Tinker, M. Tim; Guimarães, Paulo R.; Novak, Mark; Marquitti, Flavia Maria Darcie; Bodkin, James L.; Staedler, Michelle; Bentall, Gena B.; Estes, James A.

2012-01-01

Studies of consumer-resource interactions suggest that individual diet specialisation is empirically widespread and theoretically important to the organisation and dynamics of populations and communities. We used weighted networks to analyze the resource use by sea otters, testing three alternative models for how individual diet specialisation may arise. As expected, individual specialisation was absent when otter density was low, but increased at high-otter density. A high-density emergence of nested resource-use networks was consistent with the model assuming individuals share preference ranks. However, a density-dependent emergence of a non-nested modular network for ‘core’ resources was more consistent with the ‘competitive refuge’ model. Individuals from different diet modules showed predictable variation in rank-order prey preferences and handling times of core resources, further supporting the competitive refuge model. Our findings support a hierarchical organisation of diet specialisation and suggest individual use of core and marginal resources may be driven by different selective pressures.

Ion acceleration in electrostatic collisionless shock: on the optimal density profile for quasi-monoenergetic beams

DOE PAGES

Boella, E.; Fiúza, F.; Novo, A. Stockem; ...

2018-02-01

Here, a numerical study on ion acceleration in electrostatic shock waves is presented, with the aim of determining the best plasma configuration to achieve quasi-monoenergetic ion beams in laser-driven systems. It was recently shown that tailored near-critical density plasmas characterized by a long-scale decreasing rear density profile lead to beams with low energy spread (Fiúza et al 2012 Phys. Rev. Lett. 109 215001). In this work, a detailed parameter scan investigating different plasma scale lengths is carried out. As result, the optimal plasma spatial scale length that allows for minimizing the energy spread while ensuring a significant reflection of ionsmore » by the shock is identified. Furthermore, a new configuration where the required profile has been obtained by coupling micro layers of different densities is proposed. Lastly, results show that this new engineered approach is a valid alternative, guaranteeing a low energy spread with a higher level of controllability.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bose, Arnab, E-mail: arnabbose@ee.iitb.ac.in; Jain, Sourabh; Asam, Nagarjuna

We report the thermally driven giant field-like spin-torque in magnetic tunnel junctions (MTJ) on application of heat current from top to bottom. The field-like term is detected by the shift of the magneto-resistance hysteresis loop applying temperature gradient. We observed that the field-like term depends on the magnetic symmetry of the MTJ. In asymmetric structures, with different ferromagnetic materials for free and fixed layers, the field-like term is greatly enhanced. Our results show that a pure spin current density of the order of 10{sup 9 }A/m{sup 2} can be produced by creating a 120 mK temperature difference across 0.9 nm thick MgOmore » tunnelling barrier. Our results will be useful for writing MTJ and domain wall-based memories using thermally driven spin torque.« less

The buoyancy-driven motion of a single skirted bubble or drop rising through a viscous liquid

NASA Astrophysics Data System (ADS)

Ohta, Mitsuhiro; Sussman, Mark

2012-11-01

The buoyancy-driven motion of a single skirted bubble or drop rising through a viscous liquid is computationally explored by way of 3d-axisymmetric computations. The Navier-Stokes equations for incompressible two-fluid flow are solved numerically in which the coupled level-set and volume-of-fluid method is used to simulate the deforming bubble/drop boundary and the interface jump conditions on the deforming boundary are enforced through a sharp interface numerical treatment. Dynamic, block structured adaptive grid refinement is employed in order to sufficiently resolve the thin skirts. Results on the sensitivity of the thickness of trailing bubble/drop skirts to the density ratio and viscosity ratio are reported. It is shown that both the density ratio (not the density difference) and the viscosity ratio effect the skirt thickness. Previous theory for predicting skirt thickness can be refined as a result of our calculations. It is also discovered that the formation of thin skirts for bubbles and drops have little effect on the rise velocity. In other words, the measured Re number for cases without skirt formation have almost the same values for Re as cases with a thin skirt.

Modeling of Firn Compaction for Estimating Ice-Sheet Mass Change from Observed Ice-Sheet Elevation Change

NASA Technical Reports Server (NTRS)

Li, Jun; Zwally, H. Jay

2011-01-01

Changes in ice-sheet surface elevation are caused by a combination of ice-dynamic imbalance, ablation, temporal variations in accumulation rate, firn compaction and underlying bedrock motion. Thus, deriving the rate of ice-sheet mass change from measured surface elevation change requires information on the rate of firn compaction and bedrock motion, which do not involve changes in mass, and requires an appropriate firn density to associate with elevation changes induced by recent accumulation rate variability. We use a 25 year record of surface temperature and a parameterization for accumulation change as a function of temperature to drive a firn compaction model. We apply this formulation to ICESat measurements of surface elevation change at three locations on the Greenland ice sheet in order to separate the accumulation-driven changes from the ice-dynamic/ablation-driven changes, and thus to derive the corresponding mass change. Our calculated densities for the accumulation-driven changes range from 410 to 610 kg/cu m, which along with 900 kg/cu m for the dynamic/ablation-driven changes gives average densities ranging from 680 to 790 kg/cu m. We show that using an average (or "effective") density to convert elevation change to mass change is not valid where the accumulation and the dynamic elevation changes are of opposite sign.

Improved characterization of heterogeneous permeability in saline aquifers from transient pressure data during freshwater injection

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kang, Peter K.; Lee, Jonghyun; Fu, Xiaojing

Managing recharge of freshwater into saline aquifers requires accurate estimation of the heterogeneous permeability field for maximizing injection and recovery efficiency. Here we present a methodology for subsurface characterization in saline aquifers that takes advantage of the density difference between the injected freshwater and the ambient saline groundwater. We combine high-resolution forward modeling of density-driven flow with an efficient Bayesian geostatistical inversion algorithm. In the presence of a density difference between the injected and ambient fluids due to differences in salinity, the pressure field is coupled to the spatial distribution of salinity. This coupling renders the pressure field transient: themore » time evolution of the salinity distribution controls the density distribution which then leads to a time-evolving pressure distribution. We exploit this coupling between pressure and salinity to obtain an improved characterization of the permeability field without multiple pumping tests or additional salinity measurements. We show that the inversion performance improves with an increase in the mixed convection ratio—the relative importance between viscous forces from injection and buoyancy forces from density difference. Thus, our work shows that measuring transient pressure data at multiple sampling points during freshwater injection into saline aquifers can be an effective strategy for aquifer characterization, key to the successful management of aquifer recharge.« less

Improved characterization of heterogeneous permeability in saline aquifers from transient pressure data during freshwater injection

DOE PAGES

Kang, Peter K.; Lee, Jonghyun; Fu, Xiaojing; ...

2017-05-31

Managing recharge of freshwater into saline aquifers requires accurate estimation of the heterogeneous permeability field for maximizing injection and recovery efficiency. Here we present a methodology for subsurface characterization in saline aquifers that takes advantage of the density difference between the injected freshwater and the ambient saline groundwater. We combine high-resolution forward modeling of density-driven flow with an efficient Bayesian geostatistical inversion algorithm. In the presence of a density difference between the injected and ambient fluids due to differences in salinity, the pressure field is coupled to the spatial distribution of salinity. This coupling renders the pressure field transient: themore » time evolution of the salinity distribution controls the density distribution which then leads to a time-evolving pressure distribution. We exploit this coupling between pressure and salinity to obtain an improved characterization of the permeability field without multiple pumping tests or additional salinity measurements. We show that the inversion performance improves with an increase in the mixed convection ratio—the relative importance between viscous forces from injection and buoyancy forces from density difference. Thus, our work shows that measuring transient pressure data at multiple sampling points during freshwater injection into saline aquifers can be an effective strategy for aquifer characterization, key to the successful management of aquifer recharge.« less

Experimental study of linear and nonlinear regimes of density-driven instabilities induced by CO{sub 2} dissolution in water

DOE Office of Scientific and Technical Information (OSTI.GOV)

Outeda, R.; D'Onofrio, A.; El Hasi, C.

Density driven instabilities produced by CO{sub 2} (gas) dissolution in water containing a color indicator were studied in a Hele Shaw cell. The images were analyzed and instability patterns were characterized by mixing zone temporal evolution, dispersion curves, and the growth rate for different CO{sub 2} pressures and different color indicator concentrations. The results obtained from an exhaustive analysis of experimental data show that this system has a different behaviour in the linear regime of the instabilities (when the growth rate has a linear dependence with time), from the nonlinear regime at longer times. At short times using a colormore » indicator to see the evolution of the pattern, the images show that the effects of both the color indicator and CO{sub 2} pressure are of the same order of magnitude: The growth rates are similar and the wave numbers are in the same range (0–30 cm{sup −1}) when the system is unstable. Although in the linear regime the dynamics is affected similarly by the presence of the indicator and CO{sub 2} pressure, in the nonlinear regime, the influence of the latter is clearly more pronounced than the effects of the color indicator.« less

Intelligence and EEG current density using low-resolution electromagnetic tomography (LORETA).

PubMed

Thatcher, R W; North, D; Biver, C

2007-02-01

The purpose of this study was to compare EEG current source densities in high IQ subjects vs. low IQ subjects. Resting eyes closed EEG was recorded from 19 scalp locations with a linked ears reference from 442 subjects ages 5 to 52 years. The Wechsler Intelligence Test was administered and subjects were divided into low IQ (< or =90), middle IQ (>90 to <120) and high IQ (> or =120) groups. Low-resolution electromagnetic tomographic current densities (LORETA) from 2,394 cortical gray matter voxels were computed from 1-30 Hz based on each subject's EEG. Differences in current densities using t tests, multivariate analyses of covariance, and regression analyses were used to evaluate the relationships between IQ and current density in Brodmann area groupings of cortical gray matter voxels. Frontal, temporal, parietal, and occipital regions of interest (ROIs) consistently exhibited a direct relationship between LORETA current density and IQ. Maximal t test differences were present at 4 Hz, 9 Hz, 13 Hz, 18 Hz, and 30 Hz with different anatomical regions showing different maxima. Linear regression fits from low to high IQ groups were statistically significant (P < 0.0001). Intelligence is directly related to a general level of arousal and to the synchrony of neural populations driven by thalamo-cortical resonances. A traveling frame model of sequential microstates is hypothesized to explain the results.

Impacts of the Affordable Care Act's Medicaid Expansion on Women of Reproductive Age: Differences by Parental Status and State Policies.

PubMed

Johnston, Emily M; Strahan, Andrea E; Joski, Peter; Dunlop, Anne L; Adams, E Kathleen

We use data from the Behavioral Risk Factor Surveillance System (BRFSS) from 2012 to 2015 to estimate the effects of the Affordable Care Act's (ACA) Medicaid expansions on insurance coverage and access to care for low-income women of reproductive age (19-44). We use two-way fixed effects difference-in-differences models to estimate the effects of Medicaid expansions on low-income (<100% of the Federal Poverty Level) women of reproductive age. Additional models are stratified to estimate effects based on women's parental status, pre-ACA state Medicaid eligibility levels, and the presence of a state Medicaid family planning waiver. ACA Medicaid expansions decreased uninsurance among low-income women of reproductive age by 13.2 percentage points. This decrease was driven by a decrease of 27.4 percentage points for women without dependent children, who also experienced a decrease in the likelihood of not having a personal doctor (13.3 percentage points). We find a 3.8-percentage point reduction in the likelihood of experiencing a cost barrier to care among all women, but no significant effects for other access measures or subgroups. When stratified by state policies, decreases in uninsurance were greater in states expanding from pre-ACA eligibility levels of less than 50% of Federal Poverty Level (19.4 percentage points) and in states without a Medicaid family planning waiver (17.6 percentage points). The ACA Medicaid expansion increased insurance coverage for low-income women of reproductive age, with the greatest effects for women without dependent children and women residing in states with relatively lower pre-ACA Medicaid eligibility levels or with no family planning waiver before the ACA. Copyright © 2017 Jacobs Institute of Women's Health. Published by Elsevier Inc. All rights reserved.

Gender equality and smoking: a theory-driven approach to smoking gender differences in Spain.

PubMed

Bilal, Usama; Beltrán, Paula; Fernández, Esteve; Navas-Acien, Ana; Bolumar, Francisco; Franco, Manuel

2016-05-01

The intersection between gender and class can aid in understanding gender differences in smoking. To analyse how changes in gender inequality relate to differences in smoking prevalence by gender, education and birth cohort in Spain over the past five decades (1960-2010). The Gender Inequality Index (GII) was calculated in 5-year intervals from 1960 to 2010. GII ranges from 0 to 1 (1=highest inequality) and encompasses three dimensions: reproductive health, empowerment and labour market. Estimates of female and male smoking prevalence were reconstructed from representative National Health Surveys and stratified by birth cohort and level of education. We calculated female-to-male smoking ratios from 1960 to 2010 stratified by education and birth cohort. Gender inequality in Spain decreased from 0.65 to 0.09 over the past 50 years. This rapid decline was inversely correlated (r=-0.99) to a rising female-to-male smoking ratio. The youngest birth cohort of the study (born 1980-1990) and women with high education levels had similar smoking prevalences compared with men. Women with high levels of education were also the first to show a reduction in smoking prevalence, compared with less educated women. Gender inequality fell significantly in Spain over the past 50 years. This process was accompanied by converging trends in smoking prevalence for men and women. Smoking prevalence patterns varied greatly by birth cohort and education levels. Countries in earlier stages of the tobacco epidemic should consider gender-sensitive tobacco control measures and policies. Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://www.bmj.com/company/products-services/rights-and-licensing/

WHAT DOES A SUBMILLIMETER GALAXY SELECTION ACTUALLY SELECT? THE DEPENDENCE OF SUBMILLIMETER FLUX DENSITY ON STAR FORMATION RATE AND DUST MASS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hayward, Christopher C.; Keres, Dusan; Jonsson, Patrik

2011-12-20

We perform three-dimensional dust radiative transfer (RT) calculations on hydrodynamic simulations of isolated and merging disk galaxies in order to quantitatively study the dependence of observed-frame submillimeter (submm) flux density on galaxy properties. We find that submm flux density and star formation rate (SFR) are related in dramatically different ways for quiescently star-forming galaxies and starbursts. Because the stars formed in the merger-induced starburst do not dominate the bolometric luminosity and the rapid drop in dust mass and more compact geometry cause a sharp increase in dust temperature during the burst, starbursts are very inefficient at boosting submm flux densitymore » (e.g., a {approx}> 16 Multiplication-Sign boost in SFR yields a {approx}< 2 Multiplication-Sign boost in submm flux density). Moreover, the ratio of submm flux density to SFR differs significantly between the two modes; thus one cannot assume that the galaxies with highest submm flux density are necessarily those with the highest bolometric luminosity or SFR. These results have important consequences for the bright submillimeter-selected galaxy (SMG) population. Among them are: (1) The SMG population is heterogeneous. In addition to merger-driven starbursts, there is a subpopulation of galaxy pairs, where two disks undergoing a major merger but not yet strongly interacting are blended into one submm source because of the large ({approx}> 15'' or {approx}130 kpc at z = 2) beam of single-dish submm telescopes. (2) SMGs must be very massive (M{sub *} {approx}> 6 Multiplication-Sign 10{sup 10} M{sub Sun }). (3) The infall phase makes the SMG duty cycle a factor of a few greater than what is expected for a merger-driven starburst. Finally, we provide fitting functions for SCUBA and AzTEC submm flux densities as a function of SFR and dust mass and bolometric luminosity and dust mass; these should be useful for calculating submm flux density in semi-analytic models and cosmological simulations when performing full RT is computationally not feasible.« less

Benefit of Adding Ezetimibe to Statin Therapy on Cardiovascular Outcomes and Safety in Patients With Versus Without Diabetes Mellitus: Results From IMPROVE-IT (Improved Reduction of Outcomes: Vytorin Efficacy International Trial).

PubMed

Giugliano, Robert P; Cannon, Christopher P; Blazing, Michael A; Nicolau, José C; Corbalán, Ramón; Špinar, Jindřich; Park, Jeong-Gun; White, Jennifer A; Bohula, Erin A; Braunwald, Eugene

2018-04-10

Ezetimibe, when added to simvastatin, reduces cardiovascular events after acute coronary syndrome. We explored outcomes stratified by diabetes mellitus (DM). In IMPROVE-IT (Improved Reduction of Outcomes: Vytorin Efficacy International Trial), 18 144 patients after acute coronary syndrome with low-density lipoprotein cholesterol 50 to 125 mg/dL were randomized to 40 mg ezetimibe/simvastatin (E/S) or 40 mg placebo/simvastatin. The primary composite end point was cardiovascular death, major coronary events, and stroke. DM was a prespecified subgroup. The 4933 (27%) patients with DM were more often older and female, had had a prior myocardial infarction and revascularization, and presented more frequently with non-ST segment elevation acute coronary syndrome compared with patients without DM (each P <0.001). The median admission low-density lipoprotein cholesterol was lower among patients with DM (89 versus 97 mg/dL, P <0.001). E/S achieved a significantly lower median time-weighted average low-density lipoprotein cholesterol compared with placebo/simvastatin, irrespective of DM (DM: 49 versus 67 mg/dL; no DM: 55 versus 71 mg/dL; both P <0.001). In patients with DM, E/S reduced the 7-year Kaplan-Meier primary end point event rate by 5.5% absolute (hazard ratio, 0.85; 95% confidence interval, 0.78-0.94); in patients without DM, the absolute difference was 0.7% (hazard ratio, 0.98; 95% confidence interval, 0.91-1.04; P int =0.02). The largest relative reductions in patients with DM were in myocardial infarction (24%) and ischemic stroke (39%). No differences in safety outcomes by treatment were present regardless of DM. When stratified further by age, patients ≥75 years of age had a 20% relative reduction in the primary end point regardless of DM ( P int =0.91), whereas patients <75 years of age with DM had greater benefit than those without ( P int =0.011). When stratified by the TIMI (Thrombolysis in Myocardial Infarction) Risk Score for Secondary Prevention, all patients with DM demonstrated benefit with E/S regardless of risk. In contrast, among patients without DM, those with a high risk score experienced a significant (18%) relative reduction in the composite of cardiovascular death, myocardial infarction, and ischemic stroke with E/S compared with placebo/simvastatin, whereas patients without DM at low or moderate risk demonstrated no benefit with the addition of ezetimibe to simvastatin ( P int =0.034). In IMPROVE-IT, the benefit of adding ezetimibe to statin was enhanced in patients with DM and in high-risk patients without DM. URL: https://www.clinicaltrials.gov. Unique identifier: NCT00202878. © 2017 American Heart Association, Inc.

On the fragmentation of filaments in a molecular cloud simulation

NASA Astrophysics Data System (ADS)

Chira, R.-A.; Kainulainen, J.; Ibáñez-Mejía, J. C.; Henning, Th.; Mac Low, M.-M.

2018-03-01

Context. The fragmentation of filaments in molecular clouds has attracted a lot of attention recently as there seems to be a close relation between the evolution of filaments and star formation. The study of the fragmentation process has been motivated by simple analytical models. However, only a few comprehensive studies have analysed the evolution of filaments using numerical simulations where the filaments form self-consistently as part of large-scale molecular cloud evolution. Aim. We address the early evolution of parsec-scale filaments that form within individual clouds. In particular, we focus on three questions: How do the line masses of filaments evolve? How and when do the filaments fragment? How does the fragmentation relate to the line masses of the filaments? Methods: We examine three simulated molecular clouds formed in kiloparsec-scale numerical simulations performed with the FLASH adaptive mesh refinement magnetohydrodynamic code. The simulations model a self-gravitating, magnetised, stratified, supernova-driven interstellar medium, including photoelectric heating and radiative cooling. We follow the evolution of the clouds for 6 Myr from the time self-gravity starts to act. We identify filaments using the DisPerSe algorithm, and compare the results to other filament-finding algorithms. We determine the properties of the identified filaments and compare them with the predictions of analytic filament stability models. Results: The average line masses of the identified filaments, as well as the fraction of mass in filamentary structures, increases fairly continuously after the onset of self-gravity. The filaments show fragmentation starting relatively early: the first fragments appear when the line masses lie well below the critical line mass of Ostriker's isolated hydrostatic equilibrium solution ( 16 M⊙ pc-1), commonly used as a fragmentation criterion. The average line masses of filaments identified in three-dimensional volume density cubes increases far more quickly than those identified in two-dimensional column density maps. Conclusions: Our results suggest that hydrostatic or dynamic compression from the surrounding cloud has a significant impact on the early dynamical evolution of filaments. A simple model of an isolated, isothermal cylinder may not provide a good approach for fragmentation analysis. Caution must be exercised in interpreting distributions of properties of filaments identified in column density maps, especially in the case of low-mass filaments. Comparing or combining results from studies that use different filament finding techniques is strongly discouraged.

Comparison of the key mechanisms leading to rollovers in Liquefied Natural Gas using Computational Fluid Dynamics

NASA Astrophysics Data System (ADS)

Hubert, Antoine; Dadonau, Maksim; Dembele, Siaka; Denissenko, Petr; Wen, Jennifer

2017-11-01

Growing demand for the LNG fosters growth of the number of production sites with varying composition and density. Combining different sources of LNG may result in a stably stratified system, in which heat and mass transfer between the layers is limited. Heating of the LNG due to wall thermal conductivity leads to formation of convection cells confined within the layers. While the upper layer can release the extra energy via preferential methane boil-off, the bottom layer cannot and hence becomes superheated. Gradual density equilibration reduces stratification and may eventually lead to a sudden mixing event called ``rollover'', accompanied by violent evaporation of the superheated LNG. Three phenomena are potentially responsible for density equilibration. The first is the growing difference in thermal expansion of the layers due to the reduced ability of the bottom layer to reject heat. The second is the penetration of the heated near-wall boundary layer into the upper layer. The third is the ``entrainment mixing'' occurring at the contact surface between the two layers. The present study uses CFD to compare these mechanisms. Boussinesq approximation and an extended version of the k- ɛ model is used. The code is validated by comparison with a large-scale LNG rollover experiment.

Efficient injection of radiation-pressure-accelerated sub-relativistic protons into laser wakefield acceleration based on 10 PW lasers

NASA Astrophysics Data System (ADS)

Liu, M.; Weng, S. M.; Wang, H. C.; Chen, M.; Zhao, Q.; Sheng, Z. M.; He, M. Q.; Li, Y. T.; Zhang, J.

2018-06-01

We propose a hybrid laser-driven ion acceleration scheme using a combination target of a solid foil and a density-tailored background plasma. In the first stage, a sub-relativistic proton beam can be generated by radiation pressure acceleration in intense laser interaction with the solid foil. In the second stage, this sub-relativistic proton beam is further accelerated by the laser wakefield driven by the same laser pulse in a near-critical-density background plasma with decreasing density profile. The propagating velocity of the laser front and the phase velocity of the excited wakefield wave are effectively lowered at the beginning of the second stage. By decreasing the background plasma density gradually from near critical density along the laser propagation direction, the wake travels faster and faster, while it accelerates the protons. Consequently, the dephasing between the protons and the wake is postponed and an efficient wakefield proton acceleration is achieved. This hybrid laser-driven proton acceleration scheme can be realized by using ultrashort laser pulses at the peak power of 10 PW for the generation of multi-GeV proton beams.

Ambient air pollution and emergency department visits for asthma: a multi-city assessment of effect modification by age.

PubMed

Alhanti, Brooke A; Chang, Howard H; Winquist, Andrea; Mulholland, James A; Darrow, Lyndsey A; Sarnat, Stefanie Ebelt

2016-01-01

Previous studies have found strong associations between asthma morbidity and major ambient air pollutants. Relatively little research has been conducted to assess whether age is a factor conferring susceptibility to air pollution-related asthma morbidity. We investigated the short-term relationships between asthma emergency department (ED) visits and ambient ozone (O3), carbon monoxide (CO), nitrogen dioxide (NO2), sulfur dioxide (SO2), and fine particulate matter (PM2.5) in Atlanta (1993-2009), Dallas (2006-2009), and St. Louis (2001-2007). City-specific daily time-series analyses were conducted to estimate associations by age group (0-4, 5-18, 19-39, 40-64, and 65+ years). Sub-analyses were performed stratified by race and sex. City-specific rate ratios (RRs) were combined by inverse-variance weighting to provide an overall association for each strata. The overall RRs differed across age groups, with associations for all pollutants consistently strongest for children aged 5-18 years. The patterns of association across age groups remained generally consistent when models were stratified by sex and race, although the strong observed associations among 5-18 year olds appeared to be partially driven by non-white and male patients. Our findings suggest that age is a susceptibility factor for asthma exacerbations in response to air pollution, with school-age children having the highest susceptibility.

High density laser-driven target

DOEpatents

Lindl, John D.

1981-01-01

A high density target for implosion by laser energy composed of a central quantity of fuel surrounded by a high-Z pusher shell with a low-Z ablator-pusher shell spaced therefrom forming a region filled with low-density material.

The multicategory case of the sequential Bayesian pixel selection and estimation procedure

NASA Technical Reports Server (NTRS)

Pore, M. D.; Dennis, T. B. (Principal Investigator)

1980-01-01

A Bayesian technique for stratified proportion estimation and a sampling based on minimizing the mean squared error of this estimator were developed and tested on LANDSAT multispectral scanner data using the beta density function to model the prior distribution in the two-class case. An extention of this procedure to the k-class case is considered. A generalization of the beta function is shown to be a density function for the general case which allows the procedure to be extended.

On vertical advection truncation errors in terrain-following numerical models: Comparison to a laboratory model for upwelling over submarine canyons

NASA Astrophysics Data System (ADS)

Allen, S. E.; Dinniman, M. S.; Klinck, J. M.; Gorby, D. D.; Hewett, A. J.; Hickey, B. M.

2003-01-01

Submarine canyons which indent the continental shelf are frequently regions of steep (up to 45°), three-dimensional topography. Recent observations have delineated the flow over several submarine canyons during 2-4 day long upwelling episodes. Thus upwelling episodes over submarine canyons provide an excellent flow regime for evaluating numerical and physical models. Here we compare a physical and numerical model simulation of an upwelling event over a simplified submarine canyon. The numerical model being evaluated is a version of the S-Coordinate Rutgers University Model (SCRUM). Careful matching between the models is necessary for a stringent comparison. Results show a poor comparison for the homogeneous case due to nonhydrostatic effects in the laboratory model. Results for the stratified case are better but show a systematic difference between the numerical results and laboratory results. This difference is shown not to be due to nonhydrostatic effects. Rather, the difference is due to truncation errors in the calculation of the vertical advection of density in the numerical model. The calculation is inaccurate due to the terrain-following coordinates combined with a strong vertical gradient in density, vertical shear in the horizontal velocity and topography with strong curvature.

Onset of density-driven instabilities in fractured aquifers

NASA Astrophysics Data System (ADS)

Jafari Raad, Seyed Mostafa; Hassanzadeh, Hassan

2018-04-01

Linear stability analysis is conducted to study the onset of density-driven convection involved in solubility trapping of C O2 in fractured aquifers. The effect of physical properties of a fracture network on the stability of a diffusive boundary layer in a saturated fractured porous media is investigated using the dual porosity concept. Linear stability analysis results show that both fracture interporosity flow and fracture storativity play an important role in the stability behavior of the system. It is shown that a diffusive boundary layer under the gravity field in fractured porous media with lower fracture storativity and/or higher fracture interporosity flow coefficient is more stable. We present scaling relations for the onset of convective instability in fractured aquifers with single and variable matrix block size distribution. These findings improve our understanding of density-driven flow in fractured aquifers and are important in the estimation of potential storage capacity, risk assessment, and storage site characterization and screening.

Development of a phantom to test fully automated breast density software - A work in progress.

PubMed

Waade, G G; Hofvind, S; Thompson, J D; Highnam, R; Hogg, P

2017-02-01

Mammographic density (MD) is an independent risk factor for breast cancer and may have a future role for stratified screening. Automated software can estimate MD but the relationship between breast thickness reduction and MD is not fully understood. Our aim is to develop a deformable breast phantom to assess automated density software and the impact of breast thickness reduction on MD. Several different configurations of poly vinyl alcohol (PVAL) phantoms were created. Three methods were used to estimate their density. Raw image data of mammographic images were processed using Volpara to estimate volumetric breast density (VBD%); Hounsfield units (HU) were measured on CT images; and physical density (g/cm 3 ) was calculated using a formula involving mass and volume. Phantom volume versus contact area and phantom volume versus phantom thickness was compared to values of real breasts. Volpara recognized all deformable phantoms as female breasts. However, reducing the phantom thickness caused a change in phantom density and the phantoms were not able to tolerate same level of compression and thickness reduction experienced by female breasts during mammography. Our results are promising as all phantoms resulted in valid data for automated breast density measurement. Further work should be conducted on PVAL and other materials to produce deformable phantoms that mimic female breast structure and density with the ability of being compressed to the same level as female breasts. We are the first group to have produced deformable phantoms that are recognized as breasts by Volpara software. Copyright © 2016 The College of Radiographers. All rights reserved.

On the response of the horizontal mean vertical density distribution in a fjord to low-frequency density fluctuations in the coastal water

NASA Astrophysics Data System (ADS)

Stigebrandt, Anders

1990-10-01

Baroclinic water exchange through a fjord mouth, driven by a slowly varying density field outside the mouth, is modelled by a simple quasi-steady frictionless model. It is assumed that a certain fraction of the horizontal pressure difference between the coastal water and the fjord is used to accelerate the fluid into the mouth. The continuous vertical density distribution in the fjord, which changes in response to the water exchange, is modelled using a time-dependent, one-dimensional advective-diffusive 'filling-box' type of model. The model has been tested against an almost one-year-long time series of salinity and temperature from the Ørsta fjord (horizontal surface area about 15km2) on the Norwegian west coast. It is found that for this particular fjord, the mean externally forced baroclinic water exchange is one order of magnitude greater than the mean water exchange driven by the estuarine circulation (600 and 60m3 s1 respectively). Such a vigorous water exchange between a fjord and the external area implies that the time-averaged concentrations of many biological and chemical species above the sill level in the fjord are approximately equal to those in the coastal water outside the fjords.

Buoyancy driven mixing of miscible fluids by volumetric energy deposition of microwaves.

PubMed

Wachtor, Adam J; Mocko, Veronika; Williams, Darrick J; Goertz, Matthew P; Jebrail, Farzaneh F

2013-01-01

An experiment that seeks to investigate buoyancy driven mixing of miscible fluids by microwave volumetric energy deposition is presented. The experiment involves the use of a light, non-polar fluid that initially rests on top of a heavier fluid which is more polar. Microwaves preferentially heat the polar fluid, and its density decreases due to thermal expansion. As the microwave heating continues, the density of the lower fluid eventually becomes less than that of the upper, and buoyancy driven Rayleigh-Taylor mixing ensues. The choice of fluids is crucial to the success of the experiment, and a description is given of numerous fluid combinations considered and characterized. After careful consideration, the miscible pair of toluene/tetrahydrofuran (THF) was determined as having the best potential for successful volumetric energy deposition buoyancy driven mixing. Various single fluid calibration experiments were performed to facilitate the development of a heating theory. Thereafter, results from two-fluid mixing experiments are presented that demonstrate the capability of this novel Rayleigh-Taylor driven experiment. Particular interest is paid to the onset of buoyancy driven mixing and unusual aspects of the experiment in the context of typical Rayleigh-Taylor driven mixing.

Sediment and plankton lift off recirculations in strong synthetic turbulence (KS)

NASA Astrophysics Data System (ADS)

Redondo, Jose M.; Castilla, Roberto; Sekula, Emil; Furmanek, Petr

2014-05-01

The study of particle diffusion and of turbulent sedimentation is of great importance in many geophysical fields, such as in Environmental Science or Oceanography as well as in Bio-environmental and industrial processes. For a long time, the study of diffusion was numerically computed with random free paths, which gives Brownian behavior. (Richardson 1929). These stochastics methods have the objection that do not take into account the flow profile. On the other hand, there are many ways to simulate a fluid flow, but when this is turbulent our aim is to simulate the behaviour of neutral or heavy and inertial particles of biological or geological nature in a turbulent flow, in a simple way with a kinematically simulated model and to validate the results. We use the Kinematic Simulation (KS) model, also known as Synthetic Turbulence, suggested by Kraichnan (1966) and developed further by Castilla et al.(2007), Nicolleau et al.(2012). In this model, velocity field is generated through a Fourier series of random modes. The typical scales and the energy spectrum of the turbulence are inputs of the model. As we do not solve the flow in a discrete grid, but use a random predictive expression, we can simulate the flow at the smallest scales. In an unstratified flow, a KS flow field consists of a random, truncated Fourier representation in space and time, subject to constraints associated with incompressibility, and a prescribed initial energy spectrum. For stratified calculations, two further constraints are imposed, associated with the internal wave field in stratified flows, and the tendency of density variations to suppress vertical motion. With these model modifications, good agreement is found between KS and DNS with regard to the confinement in the vertical direction characteristic of stratified turbulence. Since stratifed flows exhibit this vertical confinement, KS in strictly two dimensions was considered as a first step to understanding dispersion within a stratified flow. The properties of ensemble averages of the separation between two particles in a 2D turbulent flow were considered, and the KS approach was found to give satisfactory answers, with good comparison to experiment. We also compare structure and intermittency between KS and DNS. And experiments (Redondo 1988) The dynamical processes associated with the stably stratified atmospheric boundary layer or in the ocean thermocline are less well understood than those of its convective counterparts. This is due to its complexity, and the fact that buoyancy reduces entrainment across density interfaces. We present results on a numerical simulation of homogeneous and density stratified fluids and of comparable laboratory experiments where a sharp density interface generated by either salt concentration or heat, advances due to grid stirred turbulence Redondo (1988, 1990). The appearance of bursts of turbulence in very stable conditions due to breaking up of the internal waves, confers a sporadic character to the turbulence; these conditions of non-fully developed turbulence could explain this unusual behaviour of the scaling exponents. (Mahjoub et al. 1998, 20009 The structure functions show, in the inertial range, a potential law . The relation is concave in strong mixing situations (instability with fully developed turbulence), and convex in very stable situations (in which the breaking up of the interval waves confers a sporadic character to the turbulence).The multifractal model can not be used to represent situations of non-fully developed turbulence but the use of structure function analysis allows the investigation of intermittent and scale to scale energy transfer even in local non equilibrium flows. The relative diffusion of tracers is strongly dependent on the slope of the energy spectra which tends to Richardson's law also for very steep spectra. (Castilla et al. 2007) Local turbulence is used to establish the geometry of the turbulence mixing, changes in the equilibrium (or not) cascade may lead to more physically realistic (and understandable) models to paramerize sub-grid scaling. Care has to be taken when interpreting the direct 3D Kolmogorov cascade and the Inverse 2D Kraichnan Cascade. It is very interesting to use ESS and the third order structure functions (p=3) to investigate the scale to scale transfer of energy (and enstrophy) A parameter space based on Richardson numbers, Rossby numbers and Reynolds Numbers can be used to determine the dominant instability with different intermittencies in a complex full stratified-rotating flow. Intermittency diminishes as spectral slope increases between 5/3 (Kolmogorov's local energy balance) and 3 (Kraichnan's local enstrophy balance) like near a boundary. (Rodriguez et al 1999, Redondo et al. 1993)(Gabaldon and Redondo 2001) Helicity local balance leads to a 7/3 Energy spectra that may be strongly affected by intermittency. It should also depend on the length scale. So in K62, Kolmogorov introduced the notion of intermittency, and he would transpose the universality character of his previous constant to the universality of several parameters, the intermittence which is generalized to higher orders p, μ(p). We know that μ is not universal, as it varies from approximately 0.2 to 0.7, according to different experiments. The new energy spectra, E(k,p), has a correction term in its power: -5/3 becomes -5/3-μ(p)/9, thus, the global form of the spectra is E(k) ~ k -β(p), The different simulations produce very different spatial distributions of the bio-tracers. Gabaldon J., Redondo J.M. (2009) Plankton vertical distribution in the ocean, CUM, XTDFTG in Advances in Environmental Turbulence. UPC, Barcelona. 212. Kraichnan, R.H.: (1966), 'Dispersion of particle pairs in homogeneous turbulence', Physics Fluids, 9, 1728. Kolmogorov, A. N. (1941). The local structure of turbulence in Incompressible viscous fluid at very large Reynolds numbers. C. R. Acad. Sci. URSS 30:301. Richardson, L. F. (1929). A search forr the law of atmosferic diffusion. Beitr. Phys. frei. Atmos. 15:24. Redondo J M (1991). The structure of density interfaces, Ph. D. Thesis, CUP, University of Cambridge. Rodríguez A, Sánchez-Arcilla A, Redondo JM, Mosso C (1999) Macroturbulence measurements with electromagnetic and ultrasonic sensors: a comparison under high-turbulent flows. Exp Fluids 27:31-42 Redondo J M (1987). Effects of ground proximity on dense gas entrainment, Journal of Hazardous Materials, 16, 381-393. Redondo J M, Sanchez M A and Cantalapiedra I R (1995). Turbulent Mechanisms in Stratified Flows, Dynamics of Atmospheres and Oceans, 23, 454-462. Mahjoub O.B., Redondo J.M. and Babiano A.(1998), Structure functions in complex flows, Applied Scientific Research, 59, 299. Mahjoub O.B., Redondo J.M. and Babiano A.,(2000) Hyerarchy flux in nonhomogeneous flows in Turbulent diffusion in the environment Eds. Redondo J.M. and Babiano A. 249-260. Redondo J.M., (1988) Difusion turbulenta por rejilla oscilante. Revista de Geofisica 44, 163-174,. Vindel, J. M., Yagüe, C., & Redondo, J. M. (2008). Structure function analysis and intermittency in the atmospheric boundary layer. Nonlinear Processes in Geophysics, 15(6), 915-929.

Development of a Spot-Application Tool for Rapid, High-Resolution Simulation of Wave-Driven Nearshore Hydrodynamics

DTIC Science & Technology

2013-09-30

flow models, such as Delft3D, with our developed Boussinesq -type model. The vision of this project is to develop an operational tool for the...situ measurements or large-scale wave models. This information will be used to drive the offshore wave boundary condition. • Execute the Boussinesq ...model to match with the Boussinesq -type theory would be one which can simulate sheared and stratified currents due to large-scale (non-wave) forcings

Hierarchy of forward-backward stochastic Schrödinger equation

NASA Astrophysics Data System (ADS)

Ke, Yaling; Zhao, Yi

2016-07-01

Driven by the impetus to simulate quantum dynamics in photosynthetic complexes or even larger molecular aggregates, we have established a hierarchy of forward-backward stochastic Schrödinger equation in the light of stochastic unravelling of the symmetric part of the influence functional in the path-integral formalism of reduced density operator. The method is numerically exact and is suited for Debye-Drude spectral density, Ohmic spectral density with an algebraic or exponential cutoff, as well as discrete vibrational modes. The power of this method is verified by performing the calculations of time-dependent population differences in the valuable spin-boson model from zero to high temperatures. By simulating excitation energy transfer dynamics of the realistic full FMO trimer, some important features are revealed.

The distribution of SIgA and IgG antibody-secreting cells in the palatine tonsils of Bactrian camels (Camelus bactrianus) of different ages.

PubMed

Jia, Shuai; Zhang, Wangdong; Tan, Xuefen; He, Wanhong; Wang, Wenhui

2017-05-01

Secretory immunoglobulin A (SIgA) antibody-secreting cells (ASCs) are the major effector cells of mucosal immunity, and immunoglobulin G (IgG) ASCs are also associated with mucosal immunity. This study aimed to explore the distribution of these 2 ASC populations in the palatine tonsils of Bactrian camels of different ages. Eighteen Bactrian camels were divided into the following three age groups: pubertal (3-5 years), middle-aged (6-16 years) and old (17-20 years). SIgA and IgG ASCs within different sites of the palatine tonsils were observed through histological and immunohistochemical techniques, and their densities were analyzed using statistical methods. The results from all age groups showed that both the SIgA and IgG ASCs were primarily distributed in the subepithelial compartments of the reticulated crypt epithelium and secondarily distributed in the subepithelial compartments of the stratified surface squamous epithelium, with a few ASCs located in the extrafollicular region. Their densities in these three areas were significantly decreased in turn (P<0.05). However, the densities of SIgA ASCs were significantly higher than IgG ASCs in the same regions (P<0.05), and the densities of both ASC populations decreased with age. The results confirmed that Bactrian camel palatine tonsils are the primary mucosal immune organ producing SIgA ASCs, and the subepithelial compartment of the reticulated crypt epithelium is the primary region for the colonization and functional activity of SIgA and IgG ASCs.

Impact of the rheological layering of the lithosphere on the topography generated by sublithospheric density anomalies: Insights from analog modeling

NASA Astrophysics Data System (ADS)

Sembroni, A.; Globig, J.; Rozel, A.; Faccenna, C.; Funiciello, F.; Fernandez, M.

2013-12-01

Density anomalies located beneath the lithosphere are thought to generate dynamic topography at the surface of the Earth. Tomographic models are often used to infer the later variations of the density field in the mantle. Surface topography can then be computed using analytical solutions or numerical simulations of mantle convection. It has been shown that the viscosity profile of the upper mantle has a strong influence on the magnitude and spectral signature of surface topography and uplift rate. Here we present results from analogue modeling of the interaction between a rising ball-shaped density anomaly and the lithosphere in an isoviscous, isothermal Newtonian mantle system. Preliminary data show that surface topography is strongly influenced not only by mantle viscosity but also by density and viscosity profiles of the lithosphere. Our apparatus consists of a plexiglass square box (40x40x50 cm3) filled with glucose syrup. From the bottom a silicon ball was free to rise up until impinging a silicon plate floating on top of the syrup, mimicking the lithosphere. In order to investigate the role of lithospheric thickness and layered continental crust on stress partitioning, maximum dynamic topography, uplift rate and signal wavelength, two different configurations were tested: homogeneous lithosphere and stratified lithosphere including a low-viscosity lower crust. The topographic evolution of the surface was tracked using a laser scanning the top of the apparatus. The rise of the density anomaly was recorded by a side camera. We observe that a thick and then more resistant lithosphere makes up to 2 times lower and laterally wider topographic signatures. Layered lithospheres including a decoupling lower crust decrease the equilibrium topography and its lateral extend by ~30% to 40%. Most importantly, the uplift rate is strongly affected by the choice of lithosphere model. Both lithosphere width and the presence of a decoupling lower crust may modify the uplift rate by a factor 3. Thus, depending on the lithosphere rheology, we show that uplift rate may vary by one order of magnitude, for the same density anomaly and mantle viscosity. This result shows that surface uplift rate can be used to infer the viscosity of the upper mantle in specific Earth regions only if the rheology of the lithosphere is well constrained. With respect to previous approaches, whether numerical or analog modeling of dynamic topography, our experiments represent a new attempt to investigate the propagation of normal stresses generated by mantle flow through a rheologically stratified lithosphere and its resulting topographic signal.

Recurrence networks from multivariate signals for uncovering dynamic transitions of horizontal oil-water stratified flows

NASA Astrophysics Data System (ADS)

Gao, Zhong-Ke; Zhang, Xin-Wang; Jin, Ning-De; Donner, Reik V.; Marwan, Norbert; Kurths, Jürgen

2013-09-01

Characterizing the mechanism of drop formation at the interface of horizontal oil-water stratified flows is a fundamental problem eliciting a great deal of attention from different disciplines. We experimentally and theoretically investigate the formation and transition of horizontal oil-water stratified flows. We design a new multi-sector conductance sensor and measure multivariate signals from two different stratified flow patterns. Using the Adaptive Optimal Kernel Time-Frequency Representation (AOK TFR) we first characterize the flow behavior from an energy and frequency point of view. Then, we infer multivariate recurrence networks from the experimental data and investigate the cross-transitivity for each constructed network. We find that the cross-transitivity allows quantitatively uncovering the flow behavior when the stratified flow evolves from a stable state to an unstable one and recovers deeper insights into the mechanism governing the formation of droplets at the interface of stratified flows, a task that existing methods based on AOK TFR fail to work. These findings present a first step towards an improved understanding of the dynamic mechanism leading to the transition of horizontal oil-water stratified flows from a complex-network perspective.

Density-driven transport process following the seawater indundation of the freshwater lens of the island of Baltrum, Germany

NASA Astrophysics Data System (ADS)

Post, Vincent E. A.; Houben, Georg J.

2017-04-01

Due to the growing vulnerability of low-lying coastal zones to flooding by seawater, there is a need for more studies of the impact of inundations on fresh groundwater resources. We present previously unpublished data collected on the island of Baltrum following a devastating storm in 1962, which uniquely show the impact of seawater inundation on a freshwater lens in a siliciclastic aquifer. The field data show that elevated salinities persisted for at least 4 years at the measurement depths of 4 and 6 m, and at least for 6 years at greater depths. Numerical models confirm the importance of density-driven salt fingering. Models that did not consider density effects failed to simulate the observed breakthrough curves. Transient recharge, model dimension and lateral flow modify the details of the simulation results, but in all models density-driven flow dominates the overall system behaviour. The sequestration of intruded seawater into the deeper parts of the flow system, prolongs recovery and enhances the risk of upconing when pumping is resumed too early.

Development and validation of a turbulent-mix model for variable-density and compressible flows.

PubMed

Banerjee, Arindam; Gore, Robert A; Andrews, Malcolm J

2010-10-01

The modeling of buoyancy driven turbulent flows is considered in conjunction with an advanced statistical turbulence model referred to as the BHR (Besnard-Harlow-Rauenzahn) k-S-a model. The BHR k-S-a model is focused on variable-density and compressible flows such as Rayleigh-Taylor (RT), Richtmyer-Meshkov (RM), and Kelvin-Helmholtz (KH) driven mixing. The BHR k-S-a turbulence mix model has been implemented in the RAGE hydro-code, and model constants are evaluated based on analytical self-similar solutions of the model equations. The results are then compared with a large test database available from experiments and direct numerical simulations (DNS) of RT, RM, and KH driven mixing. Furthermore, we describe research to understand how the BHR k-S-a turbulence model operates over a range of moderate to high Reynolds number buoyancy driven flows, with a goal of placing the modeling of buoyancy driven turbulent flows at the same level of development as that of single phase shear flows.

High-Altitude Aeolian Research on the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Dong, Zhibao; Hu, Guangyin; Qian, Guangqiang; Lu, Junfeng; Zhang, Zhengcai; Luo, Wanyin; Lyu, Ping

2017-12-01

Aeolian processes and their role in desertification have been studied extensively at low elevations but have been rarely studied at high elevations in areas such as the Tibetan Plateau, where aeolian processes were active in the geologic past and remain active today. In this review, we summarize research that improves our understanding of aeolian processes on the Tibetan Plateau, including the distribution, characteristics, and provenance of aeolian sediments; the history of aeolian activity; aeolian geomorphology; and wind-driven land degradation. Contemporary aeolian processes primarily occur in dry basins, in wide river valleys, on lakeshores, on mountain slopes, and on gravel pavements. Sediment characteristics suggest a local origin and provide interesting contrasts with those of China's Loess Plateau. The history of aeolian activity and its paleoclimatic implications, reconstructed based on aeolian archives, is short (mostly since the Late Glacial) and shows wide regional differences. Aeolian geomorphology is simple and suggests short formation time. Wind-driven land degradation is less severe than previously thought, driven by different factors in different areas, and exhibited complex interactions with freeze-thaw processes. Aeolian research has been conducted within the general framework of aeolian science but addresses issues specific to the Tibetan Plateau that arise due to the low air temperature, low air density, and the presence of a cryosphere. We propose six priorities for future research: aeolian physics, the effect of freeze-thaw cycles, comparisons with other areas, regional differences, effects of wind-driven land degradation, and integrated observation and monitoring.

Electron dynamics in high energy density plasma bunch generation driven by intense picosecond laser pulse

NASA Astrophysics Data System (ADS)

Li, M.; Yuan, T.; Xu, Y. X.; Luo, S. N.

2018-05-01

When an intense picosecond laser pulse is loaded upon a dense plasma, a high energy density plasma bunch, including electron bunch and ion bunch, can be generated in the target. We simulate this process through one-dimensional particle-in-cell simulation and find that the electron bunch generation is mainly due to a local high energy density electron sphere originated in the plasma skin layer. Once generated the sphere rapidly expands to compress the surrounding electrons and induce high density electron layer, coupled with that, hot electrons are efficiently triggered in the local sphere and traveling in the whole target. Under the compressions of light pressure, forward-running and backward-running hot electrons, a high energy density electron bunch generates. The bunch energy density is as high as TJ/m3 order of magnitude in our conditions, which is significant in laser driven dynamic high pressure generation and may find applications in high energy density physics.

Correlation of breast tissue histology and optical signatures to improve margin assessment techniques

NASA Astrophysics Data System (ADS)

Kennedy, Stephanie; Caldwell, Matthew; Bydlon, Torre; Mulvey, Christine; Mueller, Jenna; Wilke, Lee; Barry, William; Ramanujam, Nimmi; Geradts, Joseph

2016-06-01

Optical spectroscopy is sensitive to morphological composition and has potential applications in intraoperative margin assessment. Here, we evaluate ex vivo breast tissue and corresponding quantified hematoxylin & eosin images to correlate optical scattering signatures to tissue composition stratified by patient characteristics. Adipose sites (213) were characterized by their cell area and density. All other benign and malignant sites (181) were quantified using a grid method to determine composition. The relationships between mean reduced scattering coefficient (<μs‧>), and % adipose, % collagen, % glands, adipocyte cell area, and adipocyte density were investigated. These relationships were further stratified by age, menopausal status, body mass index (BMI), and breast density. We identified a positive correlation between <μs‧> and % collagen and a negative correlation between <μs‧> and age and BMI. Increased collagen corresponded to increased <μs‧> variability. In postmenopausal women, <μs‧> was similar regardless of fibroglandular content. Contributions from collagen and glands to <μs‧> were independent and equivalent in benign sites; glands showed a stronger positive correlation than collagen to <μs‧> in malignant sites. Our data suggest that scattering could differentiate highly scattering malignant from benign tissues in postmenopausal women. The relationship between scattering and tissue composition will support improved scattering models and technologies to enhance intraoperative optical margin assessment.

Stratified Simulations of Collisionless Accretion Disks

NASA Astrophysics Data System (ADS)

Hirabayashi, Kota; Hoshino, Masahiro

2017-06-01

This paper presents a series of stratified-shearing-box simulations of collisionless accretion disks in the recently developed framework of kinetic magnetohydrodynamics (MHD), which can handle finite non-gyrotropy of a pressure tensor. Although a fully kinetic simulation predicted a more efficient angular-momentum transport in collisionless disks than in the standard MHD regime, the enhanced transport has not been observed in past kinetic-MHD approaches to gyrotropic pressure anisotropy. For the purpose of investigating this missing link between the fully kinetic and MHD treatments, this paper explores the role of non-gyrotropic pressure and makes the first attempt to incorporate certain collisionless effects into disk-scale, stratified disk simulations. When the timescale of gyrotropization was longer than, or comparable to, the disk-rotation frequency of the orbit, we found that the finite non-gyrotropy selectively remaining in the vicinity of current sheets contributes to suppressing magnetic reconnection in the shearing-box system. This leads to increases both in the saturated amplitude of the MHD turbulence driven by magnetorotational instabilities and in the resultant efficiency of angular-momentum transport. Our results seem to favor the fast advection of magnetic fields toward the rotation axis of a central object, which is required to launch an ultra-relativistic jet from a black hole accretion system in, for example, a magnetically arrested disk state.

The Fine Transverse Structure of a Vortex Flow Beyond the Edge of a Disc Rotating in a Stratified Fluid

NASA Astrophysics Data System (ADS)

Chashechkin, Yu. D.; Bardakov, R. N.

2018-02-01

By the methods of schlieren visualization, the evolution of elements of the fine structure of transverse vortex loops formed in the circular vortex behind the edge of a disk rotating in a continuously stratified fluid is traced for the first time. An inhomogeneous distribution of the density of a table-salt solution in a basin was formed by the continuous-squeezing method. The development of periodic perturbations at the outer boundary of the circular vortex and their transformation at the vortex-loop vertex are traced. A slow change in the angular size of the structural elements in the supercritical-flow mode is noted.

RADC Multi-Dimensional Signal-Processing Research Program.

DTIC Science & Technology

1980-09-30

Formulation 7 3.2.2 Methods of Accelerating Convergence 8 3.2.3 Application to Image Deblurring 8 3.2.4 Extensions 11 3.3 Convergence of Iterative Signal... noise -driven linear filters, permit development of the joint probability density function oz " kelihood function for the image. With an expression...spatial linear filter driven by white noise (see Fig. i). If the probability density function for the white noise is known, Fig. t. Model for image

Free Falling in Stratified Fluids

NASA Astrophysics Data System (ADS)

Lam, Try; Vincent, Lionel; Kanso, Eva

2017-11-01

Leaves falling in air and discs falling in water are examples of unsteady descents due to complex interaction between gravitational and aerodynamic forces. Understanding these descent modes is relevant to many branches of engineering and science such as estimating the behavior of re-entry space vehicles to studying biomechanics of seed dispersion. For regularly shaped objects falling in homogenous fluids, the motion is relatively well understood. However, less is known about how density stratification of the fluid medium affects the falling behavior. Here, we experimentally investigate the descent of discs in both pure water and in stable linearly stratified fluids for Froude numbers Fr 1 and Reynolds numbers Re between 1000 -2000. We found that stable stratification (1) enhances the radial dispersion of the disc at landing, (2) increases the descent time, (3) decreases the inclination (or nutation) angle, and (4) decreases the fluttering amplitude while falling. We conclude by commenting on how the corresponding information can be used as a predictive model for objects free falling in stratified fluids.

High-Beta Electromagnetic Turbulence in LAPD Plasmas

NASA Astrophysics Data System (ADS)

Rossi, G.; Carter, T. A.; Pueschel, M. J.; Jenko, F.; Told, D.; Terry, P. W.

2015-11-01

The introduction of a new LaB6 cathode plasma source in the Large Plasma Device has enabled the study of pressure-gradient-driven turbulence and transport variations at significantly higher plasma β. Density fluctuations are observed to decrease with increasing β while magnetic fluctuations increase. Furthermore, the perpendicular magnetic fluctuations are seen to saturate while parallel (compressional) magnetic fluctuations increase continuously with β. These observations are compared to linear and nonlinear simulations with the GENE code. The results are consistent with the linear excitation of a Gradient-driven Drift Coupling mode (GDC) which relies on grad-B drift due to parallel magnetic fluctuations and can be driven by density or temperature gradients.

Density-stratified flow events in Great Salt Lake, Utah, USA: implications for mercury and salinity cycling

USGS Publications Warehouse

Naftz, David L.; Carling, Gregory T.; Angeroth, Cory; Freeman, Michael; Rowland, Ryan; Pazmiño, Eddy

2014-01-01

Density stratification in saline and hypersaline water bodies from throughout the world can have large impacts on the internal cycling and loading of salinity, nutrients, and trace elements. High temporal resolution hydroacoustic and physical/chemical data were collected at two sites in Great Salt Lake (GSL), a saline lake in the western USA, to understand how density stratification may influence salinity and mercury (Hg) distributions. The first study site was in a causeway breach where saline water from GSL exchanges with less saline water from a flow restricted bay. Near-surface-specific conductance values measured in water at the breach displayed a good relationship with both flow and wind direction. No diurnal variations in the concentration of dissolved (total and MeHg loadings was observed during periods of elevated salinity. The second study site was located on the bottom of GSL where movement of a high-salinity water layer, referred to as the deep brine layer (DBL), is restricted to a naturally occurring 1.5-km-wide “spillway” structure. During selected time periods in April/May, 2012, wind-induced flow reversals in a railroad causeway breach, separating Gunnison and Gilbert Bays, were coupled with high-velocity flow pulses (up to 55 cm/s) in the DBL at the spillway site. These flow pulses were likely driven by a pressure response of highly saline water from Gunnison Bay flowing into the north basin of Gilbert Bay. Short-term flow reversal events measured at the railroad causeway breach have the ability to move measurable amounts of salt and Hg from Gunnison Bay into the DBL. Future disturbance to the steady state conditions currently imposed by the railroad causeway infrastructure could result in changes to the existing chemical balance between Gunnison and Gilbert Bays. Monitoring instruments were installed at six additional sites in the DBL during October 2012 to assess impacts from any future modifications to the railroad causeway.

Kinetic energy and scalar spectra in high Rayleigh number axially homogeneous buoyancy driven turbulence

NASA Astrophysics Data System (ADS)

Pawar, Shashikant S.; Arakeri, Jaywant H.

2016-06-01

Kinetic energy and scalar spectra from the measurements in high Rayleigh number axially homogeneous buoyancy driven turbulent flow are presented. Kinetic energy and concentration (scalar) spectra are obtained from the experiments wherein density difference is created using brine and fresh water and temperature spectra are obtained from the experiments in which heat is used. Scaling of the frequency spectra of lateral and longitudinal velocity near the tube axis is closer to the Kolmogorov-Obukhov scaling, while the scalar spectra show some evidence of dual scaling, Bolgiano-Obukhov scaling followed by Obukhov-Corrsin scaling. These scalings are also observed in the corresponding second order spatial structure functions of velocity and concentration fluctuations.

Avalanches and Criticality in Driven Magnetic Skyrmions

NASA Astrophysics Data System (ADS)

Díaz, S. A.; Reichhardt, C.; Arovas, D. P.; Saxena, A.; Reichhardt, C. J. O.

2018-03-01

We show using numerical simulations that slowly driven Skyrmions interacting with random pinning move via correlated jumps or avalanches. The avalanches exhibit power-law distributions in their duration and size, and the average avalanche shape for different avalanche durations can be scaled to a universal function, in agreement with theoretical predictions for systems in a nonequilibrium critical state. A distinctive feature of Skyrmions is the influence of the nondissipative Magnus term. When we increase the ratio of the Magnus term to the damping term, a change in the universality class of the behavior occurs, the average avalanche shape becomes increasingly asymmetric, and individual avalanches exhibit motion in the direction perpendicular to their own density gradient.

Comment on the paper "Mars Express radio occultation data: A novel analysis approach" by Grandin et al. (2014)

NASA Astrophysics Data System (ADS)

Pätzold, M.; Bird, M. K.; Häusler, B.; Peter, K.; Tellmann, S.; Tyler, G. L.

2016-10-01

In their recent paper, Grandin et al. (2014) claim to have developed a novel approach, principally a ray tracing method, to analyze radio sounding data from occulted spacecraft signals by planetary atmospheres without the usual assumptions of the radio occultation inversion method of a stratified, layered, symmetric atmosphere. They apply their "new approach" to observations of the Mars Express Radio Science (MaRS) experiment and compare their resulting temperature, neutral number density, and electron density profiles with those from MaRS, claiming that there is good agreement with the observations. The fact is, however, that there are serious disagreements in the most important altitude ranges. Their temperature profile shows a 30 K shift or a 300σ (1σ standard deviation = 0.1 K for the MaRS profile near the surface) difference toward warmer temperatures at the surface when compared with MaRS, while the MaRS profile is in best agreement with the profile from the Mars Climate Data Base V5.0 (MCD V5.0). Their full temperature profile from the surface to 250 km altitude deviates significantly from the MCD V5.0 profile. Their ionospheric electron density profile is considerably different from that derived from the MaRs observations. Although Grandin et al. (2014) claim to derive the neutral number density and temperature profiles above 200 km, including the asymptotic exosphere temperature, it is simply not possible to derive this information from what is essentially noise.

Mesh Dependence on Shear Driven Boundary Layers in Stable Stratification Generated by Large Eddy-Simulation

NASA Astrophysics Data System (ADS)

Berg, Jacob; Patton, Edward G.; Sullivan, Peter S.

2017-11-01

The effect of mesh resolution and size on shear driven atmospheric boundary layers in a stable stratified environment is investigated with the NCAR pseudo-spectral LES model (J. Atmos. Sci. v68, p2395, 2011 and J. Atmos. Sci. v73, p1815, 2016). The model applies FFT in the two horizontal directions and finite differencing in the vertical direction. With vanishing heat flux at the surface and a capping inversion entraining potential temperature into the boundary layer the situation is often called the conditional neutral atmospheric boundary layer (ABL). Due to its relevance in high wind applications such as wind power meteorology, we emphasize on second order statistics important for wind turbines including spectral information. The simulations range from mesh sizes of 643 to 10243 grid points. Due to the non-stationarity of the problem, different simulations are compared at equal eddy-turnover times. Whereas grid convergence is mostly achieved in the middle portion of the ABL, statistics close to the surface of the ABL, where the presence of the ground limits the growth of the energy containing eddies, second order statistics are not converged on the studies meshes. Higher order structure functions also reveal non-Gaussian statistics highly dependent on the resolution.

Effects of slope on the formation of dunes in dilute, turbulent pyroclastic currents: May 18th, 1980 Mt. St. Helens eruption

NASA Astrophysics Data System (ADS)

Bendana, Sylvana; Brand, Brittany D.; Self, Stephen

2014-05-01

The flanks of Mt St Helens volcano (MSH) are draped with thin, cross-stratified and stratified pyroclastic density current (PDC) deposits. These are known as the proximal bedded deposits produced during the May 18th, 1980 eruption of MSH. While the concentrated portions of the afternoon PDCs followed deep topographic drainages down the steep flanks of the volcano, the dilute overriding cloud partially decoupled to develop fully dilute, turbulent PDCs on the flanks of the volcano (Beeson, D.L. 1988. Proximal Flank Facies of the May 18, 1980 Ignimbrite: Mt. St. Helens, Washington.). The deposits along the flank thus vary greatly from those found in the pumice plain, which are generally thick, massive, poorly-sorted, block-rich deposits associated with the more concentrated portions of the flow (Brand et al, accepted. Dynamics of pyroclastic density currents: Conditions that promote substrate erosion and self-channelization - Mount St Helens, Washington (USA). JVGR). We explore the influence of topography on the formation of these dilute currents and influence of slope on the currents transport and depositional mechanisms. The deposits on steeper slopes (>15°) are fines depleted relative to the proximal bedded deposits on shallower slopes (<15°). Bedform amplitude and wavelength increase with increasing slope, as does the occurrence of regressive dunes. Increasing slope causes an increase in flow velocity and thus an increase in flow turbulence. The fines depleted deposits suggest that fine ash elutriation is more efficient in flows with stronger turbulence. The longer wavelength and amplitudes suggest that bedform morphology is directly related to flow velocity, an important finding since the controls on bedform wavelength and amplitude in density stratified flows remains poorly constrained. The occurrence of regressive dunes, often interpreted as high flow-regime bedforms, on steeper slopes relative to progressive dunes on shallower slopes further attests to the control of velocity and flow regime on bedform morphology. Samples collected from recently exposed deposits and analyzed by grain size measurements, density analyses, and crystal morphoscopy studies further assess modes of origin and transport of dilute PDCs.

A numerical study of variable density flow and mixing in porous media

NASA Astrophysics Data System (ADS)

Fan, Yin; Kahawita, René

1994-10-01

A numerical study of a negatively buoyant plume intruding into a neutrally stratified porous medium has been undertaken using finite different methods. Of particular interest has been to ascertain whether the experimentally observed gravitational instabilities that form along the lower edge of the plume are reproduced in the numerical model. The model has been found to faithfully reproduce the mean flow as well as the gravitational instabilities in the intruding plume. A linear stability analysis has confirmed the fact that the negatively buoyant plume is in fact gravitationally unstable and that the stability depends on two parameters: a concentration Rayleigh number and a characteristic length scale which is dependent on the transverse dispersivity.

Resistance noise spectroscopy across the thermally and electrically driven metal-insulator transitions in VO2 nanobeams

NASA Astrophysics Data System (ADS)

Alsaqqa, Ali; Kilcoyne, Colin; Singh, Sujay; Horrocks, Gregory; Marley, Peter; Banerjee, Sarbajit; Sambandamurthy, G.

Vanadium dioxide (VO2) is a strongly correlated material that exhibits a sharp thermally driven metal-insulator transition at Tc ~ 340 K. The transition can also be triggered by a DC voltage in the insulating phase with a threshold (Vth) behavior. The mechanisms behind these transitions are hotly discussed and resistance noise spectroscopy is a suitable tool to delineate different transport mechanisms in correlated systems. We present results from a systematic study of the low frequency (1 mHz < f < 10 Hz) noise behavior in VO2 nanobeams across the thermally and electrically driven transitions. In the thermal transition, the power spectral density (PSD) of the resistance noise is unchanged as we approach Tc from 300 K and an abrupt drop in the magnitude is seen above Tc and it remains unchanged till 400 K. However, the noise behavior in the electrically driven case is distinctly different: as the voltage is ramped from zero, the PSD gradually increases by an order of magnitude before reaching Vth and an abrupt increase is seen at Vth. The noise magnitude decreases above Vth, approaching the V = 0 value. The individual roles of percolation, Joule heating and signatures of correlated behavior will be discussed. This work is supported by NSF DMR 0847324.

Intensity and angle-of-arrival spectra of laser light propagating through axially homogeneous buoyancy-driven turbulence.

PubMed

Pawar, Shashikant S; Arakeri, Jaywant H

2016-08-01

Frequency spectra obtained from the measurements of light intensity and angle of arrival (AOA) of parallel laser light propagating through the axially homogeneous, axisymmetric buoyancy-driven turbulent flow at high Rayleigh numbers in a long (length-to-diameter ratio of about 10) vertical tube are reported. The flow is driven by an unstable density difference created across the tube ends using brine and fresh water. The highest Rayleigh number is about 8×10⁹. The aim of the present work is to find whether the conventional Obukhov-Corrsin scaling or Bolgiano-Obukhov (BO) scaling is obtained for the intensity and AOA spectra in the case of light propagation in a buoyancy-driven turbulent medium. Theoretical relations for the frequency spectra of log amplitude and AOA fluctuations developed for homogeneous isotropic turbulent media are modified for the buoyancy-driven flow in the present case to obtain the asymptotic scalings for the high and low frequency ranges. For low frequencies, the spectra of intensity and vertical AOA fluctuations obtained from measurements follow BO scaling, while scaling for the spectra of horizontal AOA fluctuations shows a small departure from BO scaling.

Internal Wave Generation by Convection

NASA Astrophysics Data System (ADS)

Lecoanet, Daniel Michael

In nature, it is not unusual to find stably stratified fluid adjacent to convectively unstable fluid. This can occur in the Earth's atmosphere, where the troposphere is convective and the stratosphere is stably stratified; in lakes, where surface solar heating can drive convection above stably stratified fresh water; in the oceans, where geothermal heating can drive convection near the ocean floor, but the water above is stably stratified due to salinity gradients; possible in the Earth's liquid core, where gradients in thermal conductivity and composition diffusivities maybe lead to different layers of stable or unstable liquid metal; and, in stars, as most stars contain at least one convective and at least one radiative (stably stratified) zone. Internal waves propagate in stably stratified fluids. The characterization of the internal waves generated by convection is an open problem in geophysical and astrophysical fluid dynamics. Internal waves can play a dynamically important role via nonlocal transport. Momentum transport by convectively excited internal waves is thought to generate the quasi-biennial oscillation of zonal wind in the equatorial stratosphere, an important physical phenomenon used to calibrate global climate models. Angular momentum transport by convectively excited internal waves may play a crucial role in setting the initial rotation rates of neutron stars. In the last year of life of a massive star, convectively excited internal waves may transport even energy to the surface layers to unbind them, launching a wind. In each of these cases, internal waves are able to transport some quantity--momentum, angular momentum, energy--across large, stable buoyancy gradients. Thus, internal waves represent an important, if unusual, transport mechanism. This thesis advances our understanding of internal wave generation by convection. Chapter 2 provides an underlying theoretical framework to study this problem. It describes a detailed calculation of the internal gravity wave spectrum, using the Lighthill theory of wave excitation by turbulence. We use a Green's function approach, in which we convolve a convective source term with the Green's function of different internal gravity waves. The remainder of the thesis is a circuitous attempt to verify these analytical predictions. I test the predictions of Chapter 2 via numerical simulation. The first step is to identify a code suitable for this study. I helped develop the Dedalus code framework to study internal wave generation by convection. Dedalus can solve many different partial differential equations using the pseudo-spectral numerical method. In Chapter 3, I demonstrate Dedalus' ability to solve different equations used to model convection in astrophysics. I consider both the propagation and damping of internal waves, and the properties of low Rayleigh number convective steady states, in six different equation sets used in the astrophysics literature. This shows that Dedalus can be used to solve the equations of interest. Next, in Chapter 4, I verify the high accuracy of Dedalus by comparing it to the popular astrophysics code Athena in a standard Kelvin-Helmholtz instability test problem. Dedalus performs admirably in comparison to Athena, and provides a high standard for other codes solving the fully compressible Navier-Stokes equations. Chapter 5 demonstrates that Dedalus can simulate convective adjacent to a stably stratified region, by studying convective mixing near carbon flames. The convective overshoot and mixing is well-resolved, and is able to generate internal waves. Confident in Dedalus' ability to study the problem at hand, Chapter 6 describes simulations inspired by water experiments of internal wave generation by convection. The experiments exploit water's unusual property that its density maximum is at 4°C, rather than at 0°C. We use a similar equation of state in Dedalus, and study internal gravity waves generation by convection in a water-like fluid. We test two models of wave generation: bulk excitation (equivalent to the Lighthill theory described in Chapter 2), and surface excitation. We find the bulk excitation model accurately reproduces the waves generated in the simulations, validating the calculations of Chapter 2.

Epithelial, metabolic and innate immunity transcriptomic signatures differentiating the rumen from other sheep and mammalian gastrointestinal tract tissues.

PubMed

Xiang, Ruidong; Oddy, Victor Hutton; Archibald, Alan L; Vercoe, Phillip E; Dalrymple, Brian P

2016-01-01

Background. Ruminants are successful herbivorous mammals, in part due to their specialized forestomachs, the rumen complex, which facilitates the conversion of feed to soluble nutrients by micro-organisms. Is the rumen complex a modified stomach expressing new epithelial (cornification) and metabolic programs, or a specialised stratified epithelium that has acquired new metabolic activities, potentially similar to those of the colon? How has the presence of the rumen affected other sections of the gastrointestinal tract (GIT) of ruminants compared to non-ruminants? Methods. Transcriptome data from 11 tissues covering the sheep GIT, two stratified epithelial and two control tissues, was analysed using principal components to cluster tissues based on gene expression profile similarity. Expression profiles of genes along the sheep GIT were used to generate a network to identify genes enriched for expression in different compartments of the GIT. The data from sheep was compared to similar data sets from two non-ruminants, pigs (closely related) and humans (more distantly related). Results. The rumen transcriptome clustered with the skin and tonsil, but not the GIT transcriptomes, driven by genes from the epidermal differentiation complex, and genes encoding stratified epithelium keratins and innate immunity proteins. By analysing all of the gene expression profiles across tissues together 16 major clusters were identified. The strongest of these, and consistent with the high turnover rate of the GIT, showed a marked enrichment of cell cycle process genes (P = 1.4 E-46), across the whole GIT, relative to liver and muscle, with highest expression in the caecum followed by colon and rumen. The expression patterns of several membrane transporters (chloride, zinc, nucleosides, amino acids, fatty acids, cholesterol and bile acids) along the GIT was very similar in sheep, pig and humans. In contrast, short chain fatty acid uptake and metabolism appeared to be different between the species and different between the rumen and colon in sheep. The importance of nitrogen and iodine recycling in sheep was highlighted by the highly preferential expression of SLC14A1-urea (rumen), RHBG-ammonia (intestines) and SLC5A5-iodine (abomasum). The gene encoding a poorly characterized member of the maltase-glucoamylase family (MGAM2), predicted to play a role in the degradation of starch or glycogen, was highly expressed in the small and large intestines. Discussion. The rumen appears to be a specialised stratified cornified epithelium, probably derived from the oesophagus, which has gained some liver-like and other specialized metabolic functions, but probably not by expression of pre-existing colon metabolic programs. Changes in gene transcription downstream of the rumen also appear have occurred as a consequence of the evolution of the rumen and its effect on nutrient composition flowing down the GIT.

Epithelial, metabolic and innate immunity transcriptomic signatures differentiating the rumen from other sheep and mammalian gastrointestinal tract tissues

PubMed Central

Xiang, Ruidong; Oddy, Victor Hutton; Archibald, Alan L.; Vercoe, Phillip E.

2016-01-01

Background. Ruminants are successful herbivorous mammals, in part due to their specialized forestomachs, the rumen complex, which facilitates the conversion of feed to soluble nutrients by micro-organisms. Is the rumen complex a modified stomach expressing new epithelial (cornification) and metabolic programs, or a specialised stratified epithelium that has acquired new metabolic activities, potentially similar to those of the colon? How has the presence of the rumen affected other sections of the gastrointestinal tract (GIT) of ruminants compared to non-ruminants? Methods. Transcriptome data from 11 tissues covering the sheep GIT, two stratified epithelial and two control tissues, was analysed using principal components to cluster tissues based on gene expression profile similarity. Expression profiles of genes along the sheep GIT were used to generate a network to identify genes enriched for expression in different compartments of the GIT. The data from sheep was compared to similar data sets from two non-ruminants, pigs (closely related) and humans (more distantly related). Results. The rumen transcriptome clustered with the skin and tonsil, but not the GIT transcriptomes, driven by genes from the epidermal differentiation complex, and genes encoding stratified epithelium keratins and innate immunity proteins. By analysing all of the gene expression profiles across tissues together 16 major clusters were identified. The strongest of these, and consistent with the high turnover rate of the GIT, showed a marked enrichment of cell cycle process genes (P = 1.4 E−46), across the whole GIT, relative to liver and muscle, with highest expression in the caecum followed by colon and rumen. The expression patterns of several membrane transporters (chloride, zinc, nucleosides, amino acids, fatty acids, cholesterol and bile acids) along the GIT was very similar in sheep, pig and humans. In contrast, short chain fatty acid uptake and metabolism appeared to be different between the species and different between the rumen and colon in sheep. The importance of nitrogen and iodine recycling in sheep was highlighted by the highly preferential expression of SLC14A1-urea (rumen), RHBG-ammonia (intestines) and SLC5A5-iodine (abomasum). The gene encoding a poorly characterized member of the maltase-glucoamylase family (MGAM2), predicted to play a role in the degradation of starch or glycogen, was highly expressed in the small and large intestines. Discussion. The rumen appears to be a specialised stratified cornified epithelium, probably derived from the oesophagus, which has gained some liver-like and other specialized metabolic functions, but probably not by expression of pre-existing colon metabolic programs. Changes in gene transcription downstream of the rumen also appear have occurred as a consequence of the evolution of the rumen and its effect on nutrient composition flowing down the GIT. PMID:26989612

Experiments with planar inductive ion source meant for creation of H+ beams.

PubMed

Vainionpaa, J H; Kalvas, T; Hahto, S K; Reijonen, J

2007-06-01

In this article the effects of different engineering parameters of rf-driven ion sources with an external spiral antenna and a quartz rf window are studied. This article consists of three main topics: the effect of source geometry on the operation gas pressure, the effect of source materials and magnetic confinement on extracted current density and ion species, and the effect of different antenna geometries on the extracted current density. The effect of source geometry was studied using three cylindrical plasma chambers with different inner diameters. The chamber materials were studied using two materials, aluminum (Al) and alumina (Al(2)O(3)). The removable 14 magnet multicusp confinement arrangement enabled us to compare the effects of the two wall materials with and without the magnetic confinement. The highest measured proton fractions were measured using Al(2)O(3) plasma chamber and no multicusp confinement. For the compared ion sources the source with multicusp confinement and Al(2)O(3) plasma chamber yields the highest current densities. Multicusp confinement increased the maximum extracted current by up to a factor of 2. Plasma production with different antenna geometries were also studied. The highest current density was achieved using 4.5 loop solenoid antenna with 6.0 cm diameter. A slightly lower current density with lower pressure was achieved using a tightly wound 3 loop spiral antenna with 3.3 cm inner diameter and 6 cm outer diameter.

Gyroscopic analogy of a rotating stratified flow confined in a tilted spheroid and its implication to stability of a heavy symmetrical top

NASA Astrophysics Data System (ADS)

Fukumoto, Yasuhide; Miyachi, Yuki

2017-11-01

We address the suppression of the gravitational instability of rotating stratified flows in a confined geometry in two ways, continuous and discontinuous stratification. A rotating flow of a stratified fluid confined in an ellipsoid, subject to gravity force, whose velocity and density fields are linear in coordinates, bears an analogy with a mechanical system of finite degrees of freedom, that is, a heavy rigid body. An insight is gained into the mechanism of system rotation for the ability of a lighter fluid of sustaining, on top of it, a heavier fluid when the angular velocity is greater than a critical value. The sleeping top corresponds to such a state. First we show that a rotating stratified flow confined in a tilted spheroid is equivalent to a heavy symmetrical top with the symmetric axis tilted from the top axis. This tilting effect of the symmetric axis on the linear stability of the sleeping top and its bifurcation is investigated in some detail. Second, we explore the incompressible two-layer RTI of a discontinuously stratified fluid confined in the lower-half of an upright spheroid rotating about the axis of symmetry oriented parallel to the vertical direction. The gyroscopic analogy accounts for decrease of the critical rotation rate with oblateness. This work was supported in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (Grant No. 16K05476).

Probabilistic density function method for nonlinear dynamical systems driven by colored noise

DOE Office of Scientific and Technical Information (OSTI.GOV)

Barajas-Solano, David A.; Tartakovsky, Alexandre M.

2016-05-01

We present a probability density function (PDF) method for a system of nonlinear stochastic ordinary differential equations driven by colored noise. The method provides an integro-differential equation for the temporal evolution of the joint PDF of the system's state, which we close by means of a modified Large-Eddy-Diffusivity-type closure. Additionally, we introduce the generalized local linearization (LL) approximation for deriving a computable PDF equation in the form of the second-order partial differential equation (PDE). We demonstrate the proposed closure and localization accurately describe the dynamics of the PDF in phase space for systems driven by noise with arbitrary auto-correlation time.more » We apply the proposed PDF method to the analysis of a set of Kramers equations driven by exponentially auto-correlated Gaussian colored noise to study the dynamics and stability of a power grid.« less

Observations of inner shelf cross-shore surface material transport adjacent to a coastal inlet in the northern Gulf of Mexico

NASA Astrophysics Data System (ADS)

Roth, Mathias K.; MacMahan, Jamie; Reniers, Ad; Özgökmen, Tamay M.; Woodall, Kate; Haus, Brian

2017-04-01

Motivated by the Deepwater Horizon oil spill, the Surfzone and Coastal Oil Pathways Experiment obtained Acoustic Doppler Current Profiler (ADCP) Eulerian and GPS-drifter based Lagrangian "surface" (<1 m) flow observations in the northern Gulf of Mexico to describe the influence of small-scale river plumes on surface material transport pathways in the nearshore. Lagrangian paths are qualitatively similar to surface pathlines derived from non-traditional, near-surface ADCP velocities, but both differ significantly from depth-averaged subsurface pathlines. Near-surface currents are linearly correlated with wind velocities (r =0.76 in the alongshore and r =0.85 in the cross-shore) at the 95% confidence level, and are 4-7 times larger than theoretical estimates of wind and wave-driven surface flow in an un-stratified water column. Differences in near-surface flow are attributed to the presence of a buoyant river plume forced by winds from passing extratropical storms. Plume boundary fronts induce a horizontal velocity gradient where drifters deployed outside of the plume in oceanic water routinely converge, slow, and are re-directed. When the plume flows west parallel to the beach, the seaward plume boundary front acts as a coastal barrier that prevents 100% of oceanic drifters from beaching within 27 km of the inlet. As a result, small-scale, wind-driven river plumes in the northern Gulf of Mexico act as coastal barriers that prevent offshore surface pollution from washing ashore west of river inlets.

Gravitational Instability of Small Particles in Stratified Dusty Disks

NASA Astrophysics Data System (ADS)

Shi, J.; Chiang, E.

2012-12-01

Self-gravity is an attractive means of forming the building blocks of planets, a.k.a. the first-generation planetesimals. For ensembles of dust particles to aggregate into self-gravitating, bound structures, they must first collect into regions of extraordinarily high density in circumstellar gas disks. We have modified the ATHENA code to simulate dusty, compressible, self-gravitating flows in a 3D shearing box configuration, working in the limit that dust particles are small enough to be perfectly entrained in gas. We have used our code to determine the critical density thresholds required for disk gas to undergo gravitational collapse. In the strict limit that the stopping times of particles in gas are infinitesimally small, our numerical simulations and analytic calculations reveal that the critical density threshold for gravitational collapse is orders of magnitude above what has been commonly assumed. We discuss how finite but still short stopping times under realistic conditions can lower the threshold to a level that may be attainable. Nonlinear development of gravitational instability in a stratified dusty disk. Shown are volume renderings of dust density for the bottom half of a disk at t=0, 6, 8, and 9 Omega^{-1}. The initial disk first develops shearing density waves. These waves then steep and form long extending filament along the azimuth. These filaments eventually break and form very dense dust clumps. The time evolution of the maximum dust density within the simulation box. Run std32 stands for a standard run which has averaged Toomre's Q=0.5. Qgtrsim 1.0 for the rest runs in the plot (Z1 has twice metallicity than the standard; Q1 has twice Q_g, the Toomre's Q for the gas disk alone; M1 has twice the dust-to-gas ratio than the standard at the midplane; R1 is constructed so that the midplane density exceeds the Roche criterion however the Toomre's Q is above unity.)

Determination of Habitat Requirements For Birds in Suburban Areas

Treesearch

Jack Ward Thomas; Richard M. DeGraaf; Joseph C. Mawson

1977-01-01

Songbird populations can be related to habitat components by a method that allows the simultaneous determination of habitat requirements for a variety of species . Through correlation and multiple-regression analyses, 10 bird species were studied in a suburban habitat, which was stratified according to human density. Variables used to account for bird distribution...

Stand density, stand structure, and species composition in transition oak stands of northwestern Pennsylvania

Treesearch

S.L. Stout

1991-01-01

Transition stands, those containing species associated with both the northern hardwood and oak-hickory forest types, are important to forest diversity in northwestern Pennsylvania. These stands have high value for a variety of forest uses, including timber production, wildlife habitat, and aesthetics. Diameter distributions are characteristically stratified by species...

Critical Dynamics of Gravito-Convective Mixing in Geological Carbon Sequestration

PubMed Central

Soltanian, Mohamad Reza; Amooie, Mohammad Amin; Dai, Zhenxue; Cole, David; Moortgat, Joachim

2016-01-01

When CO2 is injected in saline aquifers, dissolution causes a local increase in brine density that can cause Rayleigh-Taylor-type gravitational instabilities. Depending on the Rayleigh number, density-driven flow may mix dissolved CO2 throughout the aquifer at fast advective time-scales through convective mixing. Heterogeneity can impact density-driven flow to different degrees. Zones with low effective vertical permeability may suppress fingering and reduce vertical spreading, while potentially increasing transverse mixing. In more complex heterogeneity, arising from the spatial organization of sedimentary facies, finger propagation is reduced in low permeability facies, but may be enhanced through more permeable facies. The connectivity of facies is critical in determining the large-scale transport of CO2-rich brine. We perform high-resolution finite element simulations of advection-diffusion transport of CO2 with a focus on facies-based bimodal heterogeneity. Permeability fields are generated by a Markov Chain approach, which represent facies architecture by commonly observed characteristics such as volume fractions. CO2 dissolution and phase behavior are modeled with the cubic-plus-association equation-of-state. Our results show that the organization of high-permeability facies and their connectivity control the dynamics of gravitationally unstable flow. We discover new flow regimes in both homogeneous and heterogeneous media and present quantitative scaling relations for their temporal evolution. PMID:27808178

Development of a large-area planar surface-wave plasma source with a cavity launcher driven by a 915 MHz UHF wave

NASA Astrophysics Data System (ADS)

Chang, Xijiang; Kunii, Kazuki; Liang, Rongqing; Nagatsu, Masaaki

2013-04-01

A large-area planar surface-wave plasma (SWP) source driven by a 915 MHz ultrahigh frequency (UHF) wave was developed. To avoid using large, thick dielectric plates as vacuum windows, we propose a cavity launcher consisting of a cylindrical cavity with several small quartz discs at the bottom. Three types of launchers with quartz discs located at different positions were tested to compare their plasma production efficiencies and spatial distributions of electron density. With the optimum launcher, large-area plasma discharges with a radial uniformity within ±10% were obtained in a radius of about 25-30 cm in Ar gas at 8 Pa for incident power in the range 0.5-2.5 kW. The maximum electron density and temperature were approximately (0.95-1.1) × 1011 cm-3 and 1.9-2.0 eV, respectively, as measured by a Langmuir probe located 24 cm below the bottom of the cavity launcher. Using an Ar/NH3 SWP with the optimum launcher, we demonstrated large-area amino-group surface modification of polyurethane sheets. Experimental results indicated that a uniform amino-group modification was achieved over a radius of approximately 40 cm, which is slightly larger than the radial uniformity of the electron density distribution.

Advancing from Rules of Thumb: Quantifying the Effects of Small Density Changes in Mass Transport to Electrodes. Understanding Natural Convection.

PubMed

Ngamchuea, Kamonwad; Eloul, Shaltiel; Tschulik, Kristina; Compton, Richard G

2015-07-21

Understanding mass transport is prerequisite to all quantitative analysis of electrochemical experiments. While the contribution of diffusion is well understood, the influence of density gradient-driven natural convection on the mass transport in electrochemical systems is not. To date, it has been assumed to be relevant only for high concentrations of redox-active species and at long experimental time scales. If unjustified, this assumption risks misinterpretation of analytical data obtained from scanning electrochemical microscopy (SECM) and generator-collector experiments, as well as analytical sensors utilizing macroelectrodes/microelectrode arrays. It also affects the results expected from electrodeposition. On the basis of numerical simulation, herein it is demonstrated that even at less than 10 mM concentrations and short experimental times of tens of seconds, density gradient-driven natural convection significantly affects mass transport. This is evident from in-depth numerical simulation for the oxidation of hexacyanoferrate (II) at various electrode sizes and electrode orientations. In each case, the induced convection and its influence on the diffusion layer established near the electrode are illustrated by maps of the velocity fields and concentration distributions evolving with time. The effects of natural convection on mass transport and chronoamperometric currents are thus quantified and discussed for the different cases studied.

What drives the Tibetan crust to the South East Asia? Role of upper mantle density discontinuities as inferred from the continental geoid anomalies

NASA Astrophysics Data System (ADS)

Rajesh, S.

2012-04-01

The Himalaya-Tibet orogen formed as a result of the northward convergence of India into the Asia over the past 55 Ma had caused the north south crustal shortening and Cenozoic upliftment of the Tibetan plateau, which significantly affected the tectonic and climatic framework of the Asia. Geodetic measurements have also shown eastward crustal extrusion of Tibet, especially along major east-southeast strike slip faults at a slip rate of 15-20 mm a-1 and around 40 mm a-1. Such continental scale deformations have been modeled as block rotation by fault boundary stresses developed due to the India-Eurasia collision. However, the Thin Sheet model explained the crustal deformation mechanism by considering varying gravitational potential energy arise out of varying crustal thickness of the viscous lithosphere. The Channel Flow model, which also suggests extrusion is a boundary fault guided flow along the shallow crustal brittle-ductile regime. Although many models have proposed, but no consensus in these models to explain the dynamics of measured surface geodetic deformation of the Tibetan plateau. But what remains conspicuous is the origin of driving forces that cause the observed Tibetan crustal flow towards the South East Asia. Is the crustal flow originated only because of the differential stresses that developed in the shallow crustal brittle-ductile regime? Or should the stress transfer to the shallow crustal layers as a result of gravitational potential energy gradient driven upper mantle flow also to be accounted. In this work, I examine the role of latter in the light of depth distribution of continental geoid anomalies beneath the Himalaya-Tibet across major upper mantle density discontinuities. These discontinuity surfaces in the upper mantle are susceptible to hold the plastic deformation that may occur as a result of the density gradient driven flow. The distribution of geoid anomalies across these density discontinuities at 220, 410 and 660 km depth in the upper mantle beneath the Himalaya-Tibet has been studied by analyzing the geoid undulation data obtained from various satellite geodetic missions along with the recent and old (EGM2008 and EGM2006) Earth Gravity models. Results show that the net geoid anomaly varies from -65 m to -20 m, which signify a density stratified upper mantle beneath the Himalaya-Tibet and the same has been confirmed from the results of regional seismic tomography studies. The density anomaly distribution beneath Tibet from 163 km depth to its upper mantle thickness of 1063 km show a strong NW-SE elliptically oriented positive geoid anomalies of magnitude around 40 meter. Asymmetric density anomaly gradient have been observed along the Himalayan arc from west to east as well as across the arc from north to south. This caused differential gravitational potential gradient and hence an elliptical flow structure of the Tibetan continental mantle along the resultant NW-SE direction, which is in concurrence with the observed present day direction of the Tibetan crustal flow. Thus the geoid anomalies distributed at various depth ranges show how the gradient in the upper mantle gravitational potential energy, especially across the deformed discontinuity surface, is significant in determining the transfer of deviatoric stresses and providing traction to the flow of crustal layers of the Tibetan Plateau. This suggests the viscous flow model could be a preferable choice, which could better accommodate the dynamics of the upper mantle, in explaining the crustal extrusion processes of the Tibetan Plateau.

Biometric recognition via fixation density maps

NASA Astrophysics Data System (ADS)

Rigas, Ioannis; Komogortsev, Oleg V.

2014-05-01

This work introduces and evaluates a novel eye movement-driven biometric approach that employs eye fixation density maps for person identification. The proposed feature offers a dynamic representation of the biometric identity, storing rich information regarding the behavioral and physical eye movement characteristics of the individuals. The innate ability of fixation density maps to capture the spatial layout of the eye movements in conjunction with their probabilistic nature makes them a particularly suitable option as an eye movement biometrical trait in cases when free-viewing stimuli is presented. In order to demonstrate the effectiveness of the proposed approach, the method is evaluated on three different datasets containing a wide gamut of stimuli types, such as static images, video and text segments. The obtained results indicate a minimum EER (Equal Error Rate) of 18.3 %, revealing the perspectives on the utilization of fixation density maps as an enhancing biometrical cue during identification scenarios in dynamic visual environments.

Simulation model of stratified thermal energy storage tank using finite difference method

NASA Astrophysics Data System (ADS)

Waluyo, Joko

2016-06-01

Stratified TES tank is normally used in the cogeneration plant. The stratified TES tanks are simple, low cost, and equal or superior in thermal performance. The advantage of TES tank is that it enables shifting of energy usage from off-peak demand for on-peak demand requirement. To increase energy utilization in a stratified TES tank, it is required to build a simulation model which capable to simulate the charging phenomenon in the stratified TES tank precisely. This paper is aimed to develop a novel model in addressing the aforementioned problem. The model incorporated chiller into the charging of stratified TES tank system in a closed system. The model was developed in one-dimensional type involve with heat transfer aspect. The model covers the main factors affect to degradation of temperature distribution namely conduction through the tank wall, conduction between cool and warm water, mixing effect on the initial flow of the charging as well as heat loss to surrounding. The simulation model is developed based on finite difference method utilizing buffer concept theory and solved in explicit method. Validation of the simulation model is carried out using observed data obtained from operating stratified TES tank in cogeneration plant. The temperature distribution of the model capable of representing S-curve pattern as well as simulating decreased charging temperature after reaching full condition. The coefficient of determination values between the observed data and model obtained higher than 0.88. Meaning that the model has capability in simulating the charging phenomenon in the stratified TES tank. The model is not only capable of generating temperature distribution but also can be enhanced for representing transient condition during the charging of stratified TES tank. This successful model can be addressed for solving the limitation temperature occurs in charging of the stratified TES tank with the absorption chiller. Further, the stratified TES tank can be charged with the cooling energy of absorption chiller that utilizes from waste heat from gas turbine of the cogeneration plant.

Controls on Turbulent Mixing in a Strongly Stratified and Sheared Tidal River Plume

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jurisa, Joseph T.; Nash, Jonathan D.; Moum, James N.

Considerable effort has been made to parameterize turbulent kinetic energy (TKE) dissipation rate ..epsilon.. and mixing in buoyant plumes and stratified shear flows. Here, a parameterization based on Kunze et al. is examined, which estimates ..epsilon.. as the amount of energy contained in an unstable shear layer (Ri < Ric) that must be dissipated to increase the Richardson number Ri = N2/S2 to a critical value Ric within a turbulent decay time scale. Observations from the tidal Columbia River plume are used to quantitatively assess the relevant parameters controlling ..epsilon.. over a range of tidal and river discharge forcings. Observedmore » ..epsilon.. is found to be characterized by Kunze et al.'s form within a factor of 2, while exhibiting slightly decreased skill near Ri = Ric. Observed dissipation rates are compared to estimates from a constant interfacial drag formulation that neglects the direct effects of stratification. This is found to be appropriate in energetic regimes when the bulk-averaged Richardson number Rib is less than Ric/4. However, when Rib > Ric/4, the effects of stratification must be included. Similarly, ..epsilon.. scaled by the bulk velocity and density differences over the plume displays a clear dependence on Rib, decreasing as Rib approaches Ric. The Kunze et al. ..epsilon.. parameterization is modified to form an expression for the nondimensional dissipation rate that is solely a function of Rib, displaying good agreement with the observations. It is suggested that this formulation is broadly applicable for unstable to marginally unstable stratified shear flows.« less

Incorporating Density Properties of MgSO4 Brines Into Icy World Ocean Simulations

NASA Astrophysics Data System (ADS)

Goodman, J. C.; Vance, S.

2011-12-01

The structure and flow of the subsurface oceans in icy worlds depends on the sources of buoyancy within these oceans. Buoyancy is determined by the equation of state, in which density is a nonlinear function of temperature, salinity, and pressure. Equations of state for terrestrial seawater (with Na and Cl as the principal dissolved species) are well-developed, but icy world oceans may contain a different balance of species, including Na, Mg, SO4, and NH4 (Kargel et al, 2000). Recent work by Vance and Brown (2011, pers. comm.) has mapped out the density and thermodynamic properties of MgSO4 brines under icy world conditions. We have developed code to incorporate this equation of state data for MgSO4 brines into two different ocean simulation models. First, we investigate a single-column convection model, which is able to find the equilibrium structure and heat transport of an icy world ocean. We explore the heat transport through the ocean subject to a variety of assumptions about ocean salinity and seafloor heat and salt flux. We resolve the paradox posed by Vance and Brown (2004): warm salty MgSO4 brine emitted by a seafloor hydrothermal system may be positively buoyant at the seafloor, but become negatively buoyant (sinking) at lower pressure. How does heat escape the ocean, if it cannot be transported by convection? Second, we add MgSO4 dynamics to a full 3-D time-dependent general circulation model (the MIT GCM), which is able to simulate both the global-scale circulation of the world's ocean and investigate the highly turbulent dynamics of buoyant hydrothermal systems. We ask, "Are buoyancy-driven flows in a MgSO4 brine ocean significantly different than similarly-driven flows in terrestrial seawater?"

An experimental and numerical study of wave motion and upstream influence in a stratified fluid

NASA Technical Reports Server (NTRS)

Hurdis, D. A.

1974-01-01

A system consisting of two superimposed layers of liquid of different densities, with a thin transition layer at the interface, provides a good laboratory model of an ocean thermocline or of an atmospheric inversion layer. This research was to gain knowledge about the propagation of disturbances within these two geophysical systems. The technique used was to observe the propagation of internal waves and of upstream influence within the density-gradient region between the two layers of liquid. The disturbances created by the motion of a vertical flat plate, which was moved longitudinally through this region, were examined both experimentally and numerically. An upstream influence, which resulted from a balance of inertial and gravitational forces, was observed, and it was possible to predict the behavior of this influence with the numerical model. The prediction included a description of the propagation of the upstream influence to steadily increasing distances from the flat plate and the shapes and magnitudes of the velocity profiles.

Equatorial magnetic Rossby waves — evidence for a thin, strongly-buoyant stratified layer in earth's core

NASA Astrophysics Data System (ADS)

Knezek, Nicholas; Buffett, Bruce

2017-04-01

A low density stratified layer at the top of Earth's core has been proposed by many authors on the basis of chemical and thermodynamic arguments and has implications for Earth's thermal history, core energetics, and core-mantle interactions. Past studies claiming to detect a layer using perturbations in seismic wave speeds are contentious due to the extremely small magnitude of the detected signal. Recently, several studies have instead argued for the existence of a stratified layer by hypothesizing that oscillations in the observed geomagnetic field arise from waves propagating in the layer. In particular, 60 year oscillations in dipole strength have been attributed to global MAC waves, and 8 year oscillations of secular acceleration have been attributed to equatorially-trapped waves. We use a new hybrid finite-volume and Fourier numerical method we developed to model magnetohydrodynamic waves in a thin layer and show that a thin, strongly buoyant layer can produce equatorially-trapped waves with similar structures and periods to the observed 8 year signal. Using these simulated wave structures, we provide additional evidence for the existence of several propagating wave modes and place constraints on estimates for the wave periods, stratified layer thickness, and strength of buoyancy within the layer.

Magnetic Control of Solutal Buoyancy Driven Convection

NASA Technical Reports Server (NTRS)

Ramachandran, N.; Leslie, F. W.

2003-01-01

Volumetric forces resulting from local density variations and gravitational acceleration cause buoyancy induced convective motion in melts and solutions. Solutal buoyancy is a result of concentration differences in an otherwise isothermal fluid. If the fluid also exhibits variations in magnetic susceptibility with concentration then convection control by external magnetic fields can be hypothesized. Magnetic control of thermal buoyancy induced convection in ferrofluids (dispersions of ferromagnetic particles in a carrier fluid) and paramagnetic fluids have been demonstrated. Here we show the nature of magnetic control of solutal buoyancy driven convection of a paramagnetic fluid, an aqueous solution of Manganese Chloride hydrate. We predict the critical magnetic field required for balancing gravitational solutal buoyancy driven convection and validate it through a simple experiment. We demonstrate that gravity driven flow can be completely reversed by a magnetic field but the exact cancellation of the flow is not possible. This is because the phenomenon is unstable. The technique can be applied to crystal growth processes in order to reduce convection and to heat exchanger devices for enhancing convection. The method can also be applied to impose a desired g-level in reduced gravity applications.

Comparative modelling of lower hybrid current drive with two launcher designs in the Tore Supra tokamak

NASA Astrophysics Data System (ADS)

Nilsson, E.; Decker, J.; Peysson, Y.; Artaud, J.-F.; Ekedahl, A.; Hillairet, J.; Aniel, T.; Basiuk, V.; Goniche, M.; Imbeaux, F.; Mazon, D.; Sharma, P.

2013-08-01

Fully non-inductive operation with lower hybrid current drive (LHCD) in the Tore Supra tokamak is achieved using either a fully active multijunction (FAM) launcher or a more recent ITER-relevant passive active multijunction (PAM) launcher, or both launchers simultaneously. While both antennas show comparable experimental efficiencies, the analysis of stability properties in long discharges suggest different current profiles. We present comparative modelling of LHCD with the two different launchers to characterize the effect of the respective antenna spectra on the driven current profile. The interpretative modelling of LHCD is carried out using a chain of codes calculating, respectively, the global discharge evolution (tokamak simulator METIS), the spectrum at the antenna mouth (LH coupling code ALOHA), the LH wave propagation (ray-tracing code C3PO), and the distribution function (3D Fokker-Planck code LUKE). Essential aspects of the fast electron dynamics in time, space and energy are obtained from hard x-ray measurements of fast electron bremsstrahlung emission using a dedicated tomographic system. LHCD simulations are validated by systematic comparisons between these experimental measurements and the reconstructed signal calculated by the code R5X2 from the LUKE electron distribution. An excellent agreement is obtained in the presence of strong Landau damping (found under low density and high-power conditions in Tore Supra) for which the ray-tracing model is valid for modelling the LH wave propagation. Two aspects of the antenna spectra are found to have a significant effect on LHCD. First, the driven current is found to be proportional to the directivity, which depends upon the respective weight of the main positive and main negative lobes and is particularly sensitive to the density in front of the antenna. Second, the position of the main negative lobe in the spectrum is different for the two launchers. As this lobe drives a counter-current, the resulting driven current profile is also different for the FAM and PAM launchers.

Analytical solution of two-fluid electro-osmotic flows of viscoelastic fluids.

PubMed

Afonso, A M; Alves, M A; Pinho, F T

2013-04-01

This paper presents an analytical model that describes a two-fluid electro-osmotic flow of stratified fluids with Newtonian or viscoelastic rheological behavior. This is the principle of operation of an electro-osmotic two-fluid pump as proposed by Brask et al. [Tech. Proc. Nanotech., 1, 190-193, 2003], in which an electrically non-conducting fluid is transported by the interfacial dragging viscous force of a conducting fluid that is driven by electro-osmosis. The electric potential in the conducting fluid and the analytical steady flow solution of the two-fluid electro-osmotic stratified flow in a planar microchannel are presented by assuming a planar interface between the two immiscible fluids with Newtonian or viscoelastic rheological behavior. The effects of fluid rheology, shear viscosity ratio, holdup and interfacial zeta potential are analyzed to show the viability of this technique, where an enhancement of the flow rate is observed as the shear-thinning effects are increased. Copyright © 2012 Elsevier Inc. All rights reserved.

Stokesian swimming of a helical swimmer across an interface

NASA Astrophysics Data System (ADS)

Godinez, Francisco; Ramos, Armando; Zenit, Roberto

2016-11-01

Microorganisms swim in flows dominated by viscous effects but in many instances the motion occurs across heterogeneous environments where the fluid properties may vary. To our knowledge, the effect of such in-homogeneity has not been addressed in depth. We conduct experiments in which a magnetic self-propelled helical swimmer displaces across the interface between two immiscible density stratified fluids. As the swimmer crosses the interface, at a fixed rotation rate, its speed is reduced and a certain volume of the lower fluid is dragged across. We quantify the drift volume and the change of swimming speed for different swimming speeds and different fluid combinations. We relate the reduction of the swimming speed with the interfacial tension of the interface. We also compare the measurements of the drift volume with some recent calculations found in the literature.

Buoyancy-driven convection around chemical fronts traveling in covered horizontal solution layers.

PubMed

Rongy, L; Goyal, N; Meiburg, E; De Wit, A

2007-09-21

Density differences across an autocatalytic chemical front traveling horizontally in covered thin layers of solution trigger hydrodynamic flows which can alter the concentration profile. We theoretically investigate the spatiotemporal evolution and asymptotic dynamics resulting from such an interplay between isothermal chemical reactions, diffusion, and buoyancy-driven convection. The studied model couples the reaction-diffusion-convection evolution equation for the concentration of an autocatalytic species to the incompressible Stokes equations ruling the evolution of the flow velocity in a two-dimensional geometry. The dimensionless parameter of the problem is a solutal Rayleigh number constructed upon the characteristic reaction-diffusion length scale. We show numerically that the asymptotic dynamics is one steady vortex surrounding, deforming, and accelerating the chemical front. This chemohydrodynamic structure propagating at a constant speed is quite different from the one obtained in the case of a pure hydrodynamic flow resulting from the contact between two solutions of different density or from the pure reaction-diffusion planar traveling front. The dynamics is symmetric with regard to the middle of the layer thickness for positive and negative Rayleigh numbers corresponding to products, respectively, lighter or heavier than the reactants. A parametric study shows that the intensity of the flow, the propagation speed, and the deformation of the front are increasing functions of the Rayleigh number and of the layer thickness. In particular, the asymptotic mixing length and reaction-diffusion-convection speed both scale as square root Ra for Ra>5. The velocity and concentration fields in the asymptotic dynamics are also found to exhibit self-similar properties with Ra. A comparison of the dynamics in the case of a monostable versus bistable kinetics is provided. Good agreement is obtained with experimental data on the speed of iodate-arsenous acid fronts propagating in horizontal capillaries. We furthermore compare the buoyancy-driven dynamics studied here to Marangoni-driven deformation of traveling chemical fronts in solution open to the air in the absence of gravity previously studied in the same geometry [L. Rongy and A. De Wit, J. Chem. Phys. 124, 164705 (2006)].

Plasma density characterization at SPARC_LAB through Stark broadening of Hydrogen spectral lines

NASA Astrophysics Data System (ADS)

Filippi, F.; Anania, M. P.; Bellaveglia, M.; Biagioni, A.; Chiadroni, E.; Cianchi, A.; Di Giovenale, D.; Di Pirro, G.; Ferrario, M.; Mostacci, A.; Palumbo, L.; Pompili, R.; Shpakov, V.; Vaccarezza, C.; Villa, F.; Zigler, A.

2016-09-01

Plasma-based acceleration techniques are of great interest for future, compact accelerators due to their high accelerating gradient. Both particle-driven and laser-driven Plasma Wakefield Acceleration experiments are foreseen at the SPARC_LAB Test Facility (INFN National Laboratories of Frascati, Italy), with the aim to accelerate high-brightness electron beams. In order to optimize the efficiency of the acceleration in the plasma and preserve the quality of the accelerated beam, the knowledge of the plasma electron density is mandatory. The Stark broadening of the Hydrogen spectral lines is one of the candidates used to characterize plasma density. The implementation of this diagnostic for plasma-based experiments at SPARC_LAB is presented.

The emergence densities of annual cicadas (Hemiptera: Cicadidae) increase with sapling density and are greater near edges in a bottomland hardwood forest.

PubMed

Chiavacci, Scott J; Bednarz, James C; McKay, Tanja

2014-08-01

The emergence densities of cicadas tend to be patchy at multiple spatial scales. While studies have identified habitat conditions related to these patchy distributions, their interpretation has been based primarily on periodical cicada species; habitat factors associated with densities of nonperiodical (i.e., annual) cicadas have remained under studied. This is despite their widespread distribution, diversity, and role as an important trophic resource for many other organisms, particularly within riparian areas. We studied habitat factors associated with the emergence densities of Tibicen spp. in a bottomland hardwood forest in east-central Arkansas. We found emergence densities were greatest in areas of high sapling densities and increased toward forest edges, although sapling density was a much stronger predictor of emergence density. Emergence densities also differed among sample areas within our study system. The habitat features predicting nymph densities were likely driven by a combination of factors affecting female selection of oviposition sites and the effects of habitat conditions on nymph survival. The differences in nymph densities between areas of our system were likely a result of the differential effects of flooding in these areas. Interestingly, our findings were similar to observations of periodical species, suggesting that both types of cicadas select similar habitat characteristics for ovipositing or are under comparable selective pressures during development. Our findings also imply that changes in habitat characteristics because of anthropogenically altered disturbance regimes (e.g., flooding) have the potential to negatively impact both periodical and annual species, which could have dramatic consequences for organisms at numerous trophic levels.

Scaling tunable network model to reproduce the density-driven superlinear relation

NASA Astrophysics Data System (ADS)

Gao, Liang; Shan, Xiaoya; Qin, Yuhao; Yu, Senbin; Xu, Lida; Gao, Zi-You

2018-03-01

Previous works have shown the universality of allometric scaling under total and density values at the city level, but our understanding of the size effects of regions on the universality of allometric scaling remains inadequate. Here, we revisit the scaling relations between the gross domestic production (GDP) and the population based on the total and density values and first reveal that the allometric scaling under density values for different regions is universal. The scaling exponent β under the density value is in the range of (1.0, 2.0], which unexpectedly exceeds the range observed by Pan et al. [Nat. Commun. 4, 1961 (2013)]. For the wider range, we propose a network model based on a 2D lattice space with the spatial correlation factor α as a parameter. Numerical experiments prove that the generated scaling exponent β in our model is fully tunable by the spatial correlation factor α. Our model will furnish a general platform for extensive urban and regional studies.

Variability of bulk density of distillers dried grains with solubles (DDGS) during gravity-driven discharge.

PubMed

Clementson, C L; Ileleji, K E

2010-07-01

Loading railcars with consistent tonnage has immense cost implications for the shipping of distillers' dried grains with soluble (DDGS) product. Therefore, this study was designed to investigate the bulk density variability of DDGS during filling of railcar hoppers. An apparatus was developed similar to a spinning riffler sampler in order to simulate the filling of railcars at an ethanol plant. There was significant difference (P<0.05) between the initial and final measures of bulk density and particle size as the hoppers were emptied in both mass and funnel flow patterns. Particle segregation that takes place during filling of hoppers contributed to the bulk density variation and was explained by particle size variation. This phenomenon is most likely the same throughout the industry and an appropriate sampling procedure should be adopted for measuring the bulk density of DDGS stored silos or transported in railcar hoppers. Copyright (c) 2010 Elsevier Ltd. All rights reserved.

Time scales of circulation and mixing processes of San Francisco Bay waters

USGS Publications Warehouse

Walters, R.A.; Cheng, R.T.; Conomos, T.J.

1985-01-01

Conceptual models for tidal period and low-frequency variations in sea level, currents, and mixing processes in the northern and southern reaches of San Francisco Bay describe the contrasting characteristics and dissimilar processes and rates in these embayments: The northern reach is a partially mixed estuary whereas the southern reach (South Bay) is a tidally oscillating lagoon with density-driven exchanges with the northern reach. The mixed semidiurnal tides are mixtures of progressive and standing waves. The relatively simple oscillations in South Bay are nearly standing waves, with energy propagating down the channels and dispersing into the broad shoal areas. The tides of the northern reach have the general properties of a progressive wave but are altered at the constriction of the embayments and gradually change in an upstream direction to a mixture of progressive and standing waves. The spring and neap variations of the tides are pronounced and cause fortnightly varying tidal currents that affect mixing and salinity stratification in the water column. Wind stress on the water surface, freshwater inflow, and tidal currents interacting with the complex bay configuration are the major local forcing mechanisms creating low-frequency variations in sea level and currents. These local forcing mechanisms drive the residual flows which, with tidal diffusion, control the water-replacement rates in the estuary. In the northern reach, the longitudinal density gradient drives an estuarine circulation in the channels, and the spatial variation in tidal amplitude creates a tidally-driven residual circulation. In contrast, South Bay exhibits a balance between wind-driven circulation and tidally-driven residual circulation for most of the year. During winter, however, there can be sufficient density variations to drive multilayer (2 to 3) flows in the channel of South Bay. Mixing models (that include both diffusive and dispersive processes) are based on time scales associated with salt variations at the boundaries and those associated with the local forcing mechanisms, while the spatial scales of variations are dependent upon the configuration of the embayments. In the northern reach, where the estuarine circulation is strong, the salt flux is carried by the mean advection of the mean salt field. Where large salinity gradients are present, the tidal correlation part of the salt flux is of the same order as the advective part. Our knowledge of mixing and exchange rates in South Bay is poor. As this embayment is nearly isohaline, the salt flux is dominated entirely by the mean advection of the mean salt field. During and after peaks in river discharge, water mixing becomes more dynamic, with a strong density-driven current creating a net exchange of both water mass and salt. These exchanges are stronger during neap tides. Residence times of the water masses vary seasonally and differ between reaches. In the northern reach, residence times are on the order of days for high winter river discharge and of months for summer periods. The residence times for South Bay are fairly long (on the order of several months) during summer, and typically shorter (less than a month) during winter when density-driven exchanges occur. ?? 1985 Dr W. Junk Publishers.

Role of the lower hybrid spectrum in the current drive modeling for DEMO scenarios

NASA Astrophysics Data System (ADS)

Cardinali, A.; Castaldo, C.; Cesario, R.; Santini, F.; Amicucci, L.; Ceccuzzi, S.; Galli, A.; Mirizzi, F.; Napoli, F.; Panaccione, L.; Schettini, G.; Tuccillo, A. A.

2017-07-01

The active control of the radial current density profile is one of the major issues of thermonuclear fusion energy research based on magnetic confinement. The lower hybrid current drive could in principle be used as an efficient tool. However, previous understanding considered the electron temperature envisaged in a reactor at the plasma periphery too large to allow penetration of the coupled radio frequency (RF) power due to strong Landau damping. In this work, we present new numerical results based on quasilinear theory, showing that the injection of power spectra with different {n}// widths of the main lobe produce an RF-driven current density profile spanning most of the outer radial half of the plasma ({n}// is the refractive index in a parallel direction to the confinement magnetic field). Plasma kinetic profiles envisaged for the DEMO reactor are used as references. We demonstrate the robustness of the modeling results concerning the key role of the spectral width in determining the lower hybrid-driven current density profile. Scans of plasma parameters are extensively carried out with the aim of excluding the possibility that any artefact of the utilised numerical modeling would produce any novelty. We neglect here the parasitic effect of spectral broadening produced by linear scattering due to plasma density fluctuations, which mainly occurs for low magnetic field devices. This effect will be analyzed in other work that completes the report on the present breakthrough.

Development of spontaneous magnetism and half-metallicity in monolayer MoS2

NASA Astrophysics Data System (ADS)

Rahman, Altaf Ur; Rahman, Gul; García-Suárez, Víctor M.

2017-12-01

Half-metallic behavior and ferromagnetism are predicted in strained MoS2 with different light elements adsorbed using density functional theory. We find that strain increases the density of states at the Fermi energy for Y doping (Y = H, Li, and F) at the S sites and strain-driven magnetism develops in agreement with the Stoner mean field model. Strain-driven magnetism requires less strain (∼3%) for H doping as compared with F and Li doping. No saturation of the spin-magnetic moment is observed in Li-doped MoS2 due to less charge transfer from the Mo d electrons and the added atoms do not significantly increase the Spin-orbit coupling. Half-metallic ferromagnetism is predicted in H and F-doped MoS2. Fixed magnetic moments calculations are also performed, and the DFT computed data is fitted with the Landau mean field theory to investigate the emergence of spontaneous magnetism in Y-doped MoS2. We predict spontaneous magnetism in systems with large (small) mag netic moments for H/F (Li) atoms. The large (small) magnetic moments are atttributed to the electronegativity difference between S and Y atoms. These results suggest that H and F adsorbed monolayer MoS2 is a good candidate for spin-based electronic devices.

Nanometer-scale characterization of laser-driven plasmas, compression, shocks and phase transitions, by coherent small angle x-ray scattering

NASA Astrophysics Data System (ADS)

Kluge, Thomas

2015-11-01

Combining ultra-intense short-pulse and high-energy long-pulse lasers, with brilliant coherent hard X-ray FELs, such as the Helmholtz International Beamline for Extreme Fields (HIBEF) under construction at the HED Instrument of European XFEL, or MEC at LCLS, holds the promise to revolutionize our understanding of many High Energy Density Physics phenomena. Examples include the relativistic electron generation, transport, and bulk plasma response, and ionization dynamics and heating in relativistic laser-matter interactions, or the dynamics of laser-driven shocks, quasi-isentropic compression, and the kinetics of phase transitions at high pressure. A particularly promising new technique is the use of coherent X-ray diffraction to characterize electron density correlations, and by resonant scattering to characterize the distribution of specific charge-state ions, either on the ultrafast time scale of the laser interaction, or associated with hydrodynamic motion. As well one can image slight density changes arising from phase transitions inside of shock-compressed high pressure matter. The feasibility of coherent diffraction techniques in laser-driven matter will be discussed. including recent results from demonstration experiments at MEC. Among other things, very sharp density changes from laser-driven compression are observed, having an effective step width of 10 nm or smaller. This compares to a resolution of several hundred nm achievedpreviously with phase contrast imaging. and on behalf of HIBEF User Consortium, for the Helmholtz International Beamline for Extreme Fields at the European XFEL.

The deep atmosphere of Venus and the possible role of density-driven separation of CO2 and N2

NASA Astrophysics Data System (ADS)

Lebonnois, Sebastien; Schubert, Gerald

2017-07-01

With temperatures around 700 K and pressures of around 75 bar, the deepest 12 km of the atmosphere of Venus are so hot and dense that the atmosphere behaves like a supercritical fluid. The Soviet VeGa-2 probe descended through the atmosphere in 1985 and obtained the only reliable temperature profile for the deep Venusian atmosphere thus far. In this temperature profile, the atmosphere appears to be highly unstable at altitudes below 7 km, contrary to expectations. We argue that the VeGa-2 temperature profile could be explained by a change in the atmospheric gas composition, and thus molecular mass, with depth. We propose that the deep atmosphere consists of a non-homogeneous layer in which the abundance of N2--the second most abundant constituent of the Venusian atmosphere after CO2--gradually decreases to near-zero at the surface. It is difficult to explain a decline in N2 towards the surface with known nitrogen sources and sinks for Venus. Instead we suggest, partly based on experiments on supercritical fluids, that density-driven separation of N2 from CO2 can occur under the high pressures of Venus's deep atmosphere, possibly by molecular diffusion, or by natural density-driven convection. If so, the amount of nitrogen in the atmosphere of Venus is 15% lower than commonly assumed. We suggest that similar density-driven separation could occur in other massive planetary atmospheres.

Laboratory flow experiments for visualizing carbon dioxide-induced, density-driven brine convection

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kneafsey, T.; Pruess, K.

2009-09-01

Injection of carbon dioxide (CO{sub 2}) into saline aquifers confined by low-permeability cap rock will result in a layer of CO{sub 2} overlying the brine. Dissolution of CO{sub 2} into the brine increases the brine density, resulting in an unstable situation in which more-dense brine overlies less-dense brine. This gravitational instability could give rise to density-driven convection of the fluid, which is a favorable process of practical interest for CO{sub 2} storage security because it accelerates the transfer of buoyant CO{sub 2} into the aqueous phase, where it is no longer subject to an upward buoyant drive. Laboratory flow visualizationmore » tests in transparent Hele-Shaw cells have been performed to elucidate the processes and rates of this CO{sub 2} solute-driven convection (CSC). Upon introduction of CO{sub 2} into the system, a layer of CO{sub 2}-laden brine forms at the CO{sub 2}-water interface. Subsequently, small convective fingers form, which coalesce, broaden, and penetrate into the test cell. Images and time-series data of finger lengths and wavelengths are presented. Observed CO{sub 2} uptake of the convection system indicates that the CO{sub 2} dissolution rate is approximately constant for each test and is far greater than expected for a diffusion-only scenario. Numerical simulations of our system show good agreement with the experiments for onset time of convection and advancement of convective fingers. There are differences as well, the most prominent being the absence of cell-scale convection in the numerical simulations. This cell-scale convection observed in the experiments is probably initiated by a small temperature gradient induced by the cell illumination.« less

Effects of Initial Conditions on Shock Driven Flows

NASA Astrophysics Data System (ADS)

Martinez, Adam A.; Mula, Swathi M.; Charonko, John; Prestridge, Kathy

2017-11-01

The spatial and temporal evolution of shock-driven, variable density flows, such as the Richtmyer Meshkov (RM) instability, are strongly influenced by the initial conditions (IC's) of the flow at the time of interaction with shockwave. We study the effects of the IC's on the Vertical Shock Tube (VST) and on flows from Mach =1.2 to Mach =9. Experiments at the VST are of an Air-SF6 (At =0.6) multimode interface. Perturbations are generated using a shear layer with a flapper plate. Planar Laser Induced Fluorescence (PLIF) is used to characterize the IC's. New experiments are occurring using the Powder Gun driver at LANL Proton Radiography (pRad) facility. Mach number up to M =9 accelerate a Xenon-Helium (At =0.94) interface that is perturbed using a membrane supported by different sized grids. This presentation focuses on how to design and characterize different types of initial conditions for experiments.

Numerical investigation of the spreading of self-excited stratified jets

NASA Technical Reports Server (NTRS)

Batcho, P. F.; Karniadakis, G. E.; Orszag, S. A.

1990-01-01

The structure and evolution of self-excited subsonic periodic arrays of jets of constant and variable density are studied using spectral-element direct numerical simulations. The governing equation of motion is presented, and a method based on spectral element discretizations appropriate for simulating arbitrarily complex geometry jets and large density variations for subsonic flows is developed. Variable density fields are found to be more unstable than the corresponding uniform density fields with much higher rms values; as a result, their spreading is also considerably larger. There is a dramatic increase in spreading after a few pairings occur. Findings presented for low and high side-momentum flux reveal a shifting of the origin of instability from the near-field to the far-field, respectively, and suggest possible routes of stabilization.

Numerical simulation of a non-equilibrium electrokinetic micro/nano fluidic mixer

NASA Astrophysics Data System (ADS)

Hadidi, H.; Kamali, R.

2016-03-01

In this study we numerically simulate a novel micromixer that utilizes vortex generation from the non-equilibrium electrokinetics near the micro/nanochannels interface. After mixing in combined pressure-driven and electroosmotic flows was compared with mixing in a pure pressure-driven flow, the superior mixing performance of the former was evident: for a specific case study, 90% mixing of two fluid streams for a short mixing length was achieved. The results of our numerical study were very similar to those of previously reported experiments. In this paper we explain the phenomenon occurring adjacent to the nano-junctions by plotting the electrical field components, the velocity contours and the concentration distribution in the micromixer. The vortices at the micro/nanochannel interface were obviously indicators of non-equilibrium behaviour in these regions. At the end, the mixing performance was evaluated by the investigation of different applied voltages, Reynolds numbers and surface charge densities using the mixing index parameter, and the results showed that more efficient mixing occurred when the applied voltage and surface charge density magnitude were increased and the Reynolds number was decreased.

Laser imprint reduction for the critical-density foam buffered target driven by a relatively strong foot pulse at early stage of laser implosions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, J. W., E-mail: li-jiwei@iapcm.ac.cn; He, X. T.; Institute of Applied Physics and Computational Mathematics, P. O. Box 8009, Beijing 100094

In order to reduce the effect of laser imprint in direct-drive ignition scheme a low-density foam buffered target has been proposed. This target is driven by a laser pulse with a low-intensity foot at the early stage of implosion, which heats the foam and elongates the thermal conduction zone between the laser absorption region and ablation front, increasing the thermal smoothing effect. In this paper, a relatively strong foot pulse is adopted to irradiate the critical-density foam buffered target. The stronger foot, near 1 × 10{sup 14 }W/cm{sup 2}, is able to drive a radiative shock in the low-density foam, which helps smoothmore » the shock and further reduce the effect of laser imprint. The radiative shock also forms a double ablation front structure between the two ablation fronts to further stabilize the hydrodynamics, achieving the similar results to a target with a high-Z dopant in the ablator. 2D analysis shows that for the critical-density foam buffered target irradiated by the strong foot pulse, the laser imprint can be reduced due to the radiative shock in the foam and an increased thermal smoothing effect. It seems viable for the critical-density foam buffered target to be driven by a relatively strong foot pulse with the goal of reducing the laser imprint and achieving better implosion symmetry in the direct-drive laser fusion.« less

Three- and Two- Dimensional Simulations of Re-shock Experiments at High Energy Densities at the National Ignition Facility

NASA Astrophysics Data System (ADS)

Wang, Ping; Raman, Kumar; MacLaren, Stephan; Huntington, Channing; Nagel, Sabrina

2016-10-01

We present simulations of recent high-energy-density (HED) re-shock experiments on the National Ignition Facility (NIF). The experiments study the Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) instability growth that occurs after successive shocks transit a sinusoidally-perturbed interface between materials of different densities. The shock tube is driven at one or both ends using indirect-drive laser cavities or hohlraums. X-ray area-backlit imaging is used to visualize the growth at different times. Our simulations are done with the three-dimensional, radiation hydrodynamics code ARES, developed at LLNL. We show the instabilitygrowth rate, inferred from the experimental radiographs, agrees well with our 2D and 3D simulations. We also discuss some 3D geometrical effects, suggested by our simulations, which could deteriorate the images at late times, unless properly accounted for in the experiment design. Work supported by U.S. Department of Energy under Contract DE- AC52-06NA27279. LLNL-ABS-680789.

Structure and properties of the egg mass of the ommastrephid squid Todarodes pacificus

PubMed Central

Puneeta, Pandey; Yamamoto, Jun; Adachi, Kohsuke; Kato, Yoshiki; Sakurai, Yasunori

2017-01-01

The Japanese flying squid, Todarodes pacificus, is thought to spawn neutrally buoyant egg masses that retain a specific location in the water column by floating at the interface between water layers of slightly different densities. It is important to understand the physical process that determines the vertical distribution of the egg masses to predict their horizontal drift in relation to embryo survival and subsequent recruitment. Here, mesocosm experiments were conducted in a 300 m3 tank by creating a thermally stratified (17–22°C) water column to obtain egg masses. A cage net methodology was developed to sustain egg masses for detailed observation. We measured the density of the egg masses of T. pacificus, and used this information to infer the vertical distribution patterns of the egg masses at the spawning grounds (Tsushima Strait, Japan). When measured separately, the density of the outer jelly of each egg mass was 2.7 σ units higher than that of the surrounding water. The outer jelly and the specific gravity of embedded individual eggs (~1.10) cause the egg masses to have very slight negative buoyancy relative to the water in which they are formed. Analysis of the vertical profile of the spawning ground showed that water density (σθ) increased sharply at ~30 m depth; thus, egg masses might settle above the pycnocline layer. In conclusion, we suggest that T. pacificus egg masses might retain their location in the water column by floating at the interface between water layers of slightly different densities, which happen to be above the pycnocline layer (actual depth varies seasonally/annually) in the Tsushima Strait between Korea and Japan. PMID:28767686

Structure and properties of the egg mass of the ommastrephid squid Todarodes pacificus.

PubMed

Puneeta, Pandey; Vijai, Dharmamony; Yamamoto, Jun; Adachi, Kohsuke; Kato, Yoshiki; Sakurai, Yasunori

2017-01-01

The Japanese flying squid, Todarodes pacificus, is thought to spawn neutrally buoyant egg masses that retain a specific location in the water column by floating at the interface between water layers of slightly different densities. It is important to understand the physical process that determines the vertical distribution of the egg masses to predict their horizontal drift in relation to embryo survival and subsequent recruitment. Here, mesocosm experiments were conducted in a 300 m3 tank by creating a thermally stratified (17-22°C) water column to obtain egg masses. A cage net methodology was developed to sustain egg masses for detailed observation. We measured the density of the egg masses of T. pacificus, and used this information to infer the vertical distribution patterns of the egg masses at the spawning grounds (Tsushima Strait, Japan). When measured separately, the density of the outer jelly of each egg mass was 2.7 σ units higher than that of the surrounding water. The outer jelly and the specific gravity of embedded individual eggs (~1.10) cause the egg masses to have very slight negative buoyancy relative to the water in which they are formed. Analysis of the vertical profile of the spawning ground showed that water density (σθ) increased sharply at ~30 m depth; thus, egg masses might settle above the pycnocline layer. In conclusion, we suggest that T. pacificus egg masses might retain their location in the water column by floating at the interface between water layers of slightly different densities, which happen to be above the pycnocline layer (actual depth varies seasonally/annually) in the Tsushima Strait between Korea and Japan.

Battery-Powered RF Pre-Ionization System for the Caltech Magnetohydrodynamically-Driven Jet Experiment: RF Discharge Properties and MHD-Driven Jet Dynamics

NASA Astrophysics Data System (ADS)

Chaplin, Vernon H.

This thesis describes investigations of two classes of laboratory plasmas with rather different properties: partially ionized low pressure radiofrequency (RF) discharges, and fully ionized high density magnetohydrodynamically (MHD)-driven jets. An RF pre-ionization system was developed to enable neutral gas breakdown at lower pressures and create hotter, faster jets in the Caltech MHD-Driven Jet Experiment. The RF plasma source used a custom pulsed 3 kW 13.56 MHz RF power amplifier that was powered by AA batteries, allowing it to safely float at 4-6 kV with the cathode of the jet experiment. The argon RF discharge equilibrium and transport properties were analyzed, and novel jet dynamics were observed. Although the RF plasma source was conceived as a wave-heated helicon source, scaling measurements and numerical modeling showed that inductive coupling was the dominant energy input mechanism. A one-dimensional time-dependent fluid model was developed to quantitatively explain the expansion of the pre-ionized plasma into the jet experiment chamber. The plasma transitioned from an ionizing phase with depressed neutral emission to a recombining phase with enhanced emission during the course of the experiment, causing fast camera images to be a poor indicator of the density distribution. Under certain conditions, the total visible and infrared brightness and the downstream ion density both increased after the RF power was turned off. The time-dependent emission patterns were used for an indirect measurement of the neutral gas pressure. The low-mass jets formed with the aid of the pre-ionization system were extremely narrow and collimated near the electrodes, with peak density exceeding that of jets created without pre-ionization. The initial neutral gas distribution prior to plasma breakdown was found to be critical in determining the ultimate jet structure. The visible radius of the dense central jet column was several times narrower than the axial current channel radius, suggesting that the outer portion of the jet must have been force free, with the current parallel to the magnetic field. The studies of non-equilibrium flows and plasma self-organization being carried out at Caltech are relevant to astrophysical jets and fusion energy research.

Studies on the ionospheric-thermospheric coupling mechanisms using SLR

NASA Astrophysics Data System (ADS)

Panzetta, Francesca; Erdogan, Eren; Bloßfeld, Mathis; Schmidt, Michael

2016-04-01

Several Low Earth Orbiters (LEOs) have been used by different research groups to model the thermospheric neutral density distribution at various altitudes performing Precise Orbit Determination (POD) in combination with satellite accelerometry. This approach is, in principle, based on satellite drag analysis, driven by the fact that the drag force is one of the major perturbing forces acting on LEOs. The satellite drag itself is physically related to the thermospheric density. The present contribution investigates the possibility to compute the thermospheric density from Satellite Laser Ranging (SLR) observations. SLR is commonly used to compute very accurate satellite orbits. As a prerequisite, a very high precise modelling of gravitational and non-gravitational accelerations is necessary. For this investigation, a sensitivity study of SLR observations to thermospheric density variations is performed using the DGFI Orbit and Geodetic parameter estimation Software (DOGS). SLR data from satellites at altitudes lower than 500 km are processed adopting different thermospheric models. The drag coefficients which describe the interaction of the satellite surfaces with the atmosphere are analytically computed in order to obtain scaling factors purely related to the thermospheric density. The results are reported and discussed in terms of estimates of scaling coefficients of the thermospheric density. Besides, further extensions and improvements in thermospheric density modelling obtained by combining a physics-based approach with ionospheric observations are investigated. For this purpose, the coupling mechanisms between the thermosphere and ionosphere are studied.

Transport of particles, drops, and small organisms in density stratified fluids

NASA Astrophysics Data System (ADS)

Ardekani, Arezoo M.; Doostmohammadi, Amin; Desai, Nikhil

2017-10-01

Sedimenting particles and motile organisms are ubiquitously found in oceans and lakes, where density stratification naturally occurs due to temperature or salinity gradients. We explore the effects of stratification on the fundamental hydrodynamics of settling particles, rising drops, and small organisms. The results of our direct numerical simulations of the sedimentation of particles show that the presence of vertical density gradients in the water column can substantially affect the settling dynamics of a particle, interaction between a pair of particles, and settling rates and microstructure of suspension of particles. We show that elongation of particles affects both the settling orientation and the settling rate of particles in stratified fluids, which will have direct consequences on the vertical flux of particulate matter and carbon flux in the ocean. We further demonstrate an unexpected effect of buoyancy, potentially affecting a broad range of processes at pycnoclines in oceans and lakes. In particular, stratification has a major effect on the flow field, energy expenditure, and nutrient uptake of small organisms. In addition, the role of stratification in pattern formation of bioconvection plumes of algal cells and in biogenic mixing is investigated. In particular, the numerical approach allows for considering the effects of background turbulence and hydrodynamic perturbations produced by swimming organisms, shedding light on the contribution of organisms in the mixing process in aqueous environments.

Associations between breast density and a panel of single nucleotide polymorphisms linked to breast cancer risk: a cohort study with digital mammography.

PubMed

Keller, Brad M; McCarthy, Anne Marie; Chen, Jinbo; Armstrong, Katrina; Conant, Emily F; Domchek, Susan M; Kontos, Despina

2015-03-18

Breast density and single-nucleotide polymorphisms (SNPs) have both been associated with breast cancer risk. To determine the extent to which these two breast cancer risk factors are associated, we investigate the association between a panel of validated SNPs related to breast cancer and quantitative measures of mammographic density in a cohort of Caucasian and African-American women. In this IRB-approved, HIPAA-compliant study, we analyzed a screening population of 639 women (250 African American and 389 Caucasian) who were tested with a validated panel assay of 12 SNPs previously associated to breast cancer risk. Each woman underwent digital mammography as part of routine screening and all were interpreted as negative. Both absolute and percent estimates of area and volumetric density were quantified on a per-woman basis using validated software. Associations between the number of risk alleles in each SNP and the density measures were assessed through a race-stratified linear regression analysis, adjusted for age, BMI, and Gail lifetime risk. The majority of SNPs were not found to be associated with any measure of breast density. SNP rs3817198 (in LSP1) was significantly associated with both absolute area (p = 0.004) and volumetric (p = 0.019) breast density in Caucasian women. In African-American women, SNPs rs3803662 (in TNRC9/TOX3) and rs4973768 (in NEK10) were significantly associated with absolute (p = 0.042) and percent (p = 0.028) volume density respectively. The majority of SNPs investigated in our study were not found to be significantly associated with breast density, even when accounting for age, BMI, and Gail risk, suggesting that these two different risk factors contain potentially independent information regarding a woman's risk to develop breast cancer. Additionally, the few statistically significant associations between breast density and SNPs were different for Caucasian versus African American women. Larger prospective studies are warranted to validate our findings and determine potential implications for breast cancer risk assessment.

Safety analysis of proposed data-driven physiologic alarm parameters for hospitalized children.

PubMed

Goel, Veena V; Poole, Sarah F; Longhurst, Christopher A; Platchek, Terry S; Pageler, Natalie M; Sharek, Paul J; Palma, Jonathan P

2016-12-01

Modification of alarm limits is one approach to mitigating alarm fatigue. We aimed to create and validate heart rate (HR) and respiratory rate (RR) percentiles for hospitalized children, and analyze the safety of replacing current vital sign reference ranges with proposed data-driven, age-stratified 5th and 95th percentile values. In this retrospective cross-sectional study, nurse-charted HR and RR data from a training set of 7202 hospitalized children were used to develop percentile tables. We compared 5th and 95th percentile values with currently accepted reference ranges in a validation set of 2287 patients. We analyzed 148 rapid response team (RRT) and cardiorespiratory arrest (CRA) events over a 12-month period, using HR and RR values in the 12 hours prior to the event, to determine the proportion of patients with out-of-range vitals based upon reference versus data-driven limits. There were 24,045 (55.6%) fewer out-of-range measurements using data-driven vital sign limits. Overall, 144/148 RRT and CRA patients had out-of-range HR or RR values preceding the event using current limits, and 138/148 were abnormal using data-driven limits. Chart review of RRT and CRA patients with abnormal HR and RR per current limits considered normal by data-driven limits revealed that clinical status change was identified by other vital sign abnormalities or clinical context. A large proportion of vital signs in hospitalized children are outside presently used norms. Safety evaluation of data-driven limits suggests they are as safe as those currently used. Implementation of these parameters in physiologic monitors may mitigate alarm fatigue. Journal of Hospital Medicine 2015;11:817-823. © 2015 Society of Hospital Medicine. © 2016 Society of Hospital Medicine.

The Role of Scale in the Development and Evolution of Stratified Shear Turbulence, Entrainment and Mixing

DTIC Science & Technology

2015-09-30

plan for the DNS runs of this project, which will be performed by using NGA (Desjardins et al., 2008), an in-house flow solver for variable density low...examined at geophysically relevant Reynolds numbers. In this project, the LES capabilities of the NGA computational framework is used. 4 REFERENCES

Low Impact of Traditional Risk Factors on Carotid Intima-Media Thickness: The ELSA-Brasil Cohort.

PubMed

Santos, Itamar S; Alencar, Airlane P; Rundek, Tatjana; Goulart, Alessandra C; Barreto, Sandhi M; Pereira, Alexandre C; Benseñor, Isabela M; Lotufo, Paulo A

2015-09-01

There is little information about how much traditional cardiovascular risk factors explain common carotid artery intima-media thickness (CCA-IMT) variance. We aimed to study to which extent CCA-IMT values are determined by traditional risk factors and which commonly used measurements of blood pressure, glucose metabolism, lipid profile, and adiposity contribute the most to this determination in the Brazilian Longitudinal Study of Adult Health (ELSA-Brasil) cohort baseline. We analyzed 9792 individuals with complete data and CCA-IMT measurements. We built multiple linear regression models using mean left and right CCA-IMT as the dependent variable. All models were stratified by sex. We also analyzed individuals stratified by 10-year coronary heart disease risk and, in separate, those with no traditional risk factors. Main models' R(2) varied between 0.141 and 0.373. The major part of the explained variance in CCA-IMT was because of age and race. Indicators of blood pressure, lipid profile, and adiposity that most frequently composed the best models were pulse pressure, low-density lipoprotein/high-density lipoprotein ratio, and neck circumference. The association between neck circumference and CCA-IMT persisted significant even after further adjustment for vessel sizes and body mass index. Indicators of glucose metabolism had smaller contribution. We found that >60% of CCA-IMT were not explained by demographic and traditional cardiovascular risk factors, which highlights the need to study novel risk factors. Pulse pressure, low-density lipoprotein/high-density lipoprotein ratio, and neck circumference were the most consistent contributors. © 2015 American Heart Association, Inc.

Goal-Oriented Probability Density Function Methods for Uncertainty Quantification

DTIC Science & Technology

2015-12-11

approximations or data-driven approaches. We investigated the accuracy of analytical tech- niques based Kubo -Van Kampen operator cumulant expansions for...analytical techniques based Kubo -Van Kampen operator cumulant expansions for Langevin equations driven by fractional Brownian motion and other noises

The Importance of Submesoscale Versus Basin-scale Processes in Driving the Subpolar Spring Phytoplankton Bloom.

NASA Astrophysics Data System (ADS)

Brody, S.; Mahadevan, A.; Lozier, M. S.

2014-12-01

The subpolar spring phytoplankton bloom has important consequences for marine ecosystems and the carbon cycle. The timing of the bloom has been conceived of as a basin-scale event: as the ocean warms, the seasonal mixed layer shoals, restricting phytoplankton to shallower depths and increasing available light to a level at which the bloom can begin. Recent studies have highlighted the importance of localized phenomena in driving the bloom initiation. Specifically, the role of lateral density gradients in generating <10km instabilities in the upper ocean, which then stratify the mixed layer before surface heating begins, has been explored with a process study model and fine-scale observations from a field program to study the North Atlantic spring bloom [1]. However, an alternative hypothesis has recently been validated at both the small scale, using the same observational data [2], and at the basin scale, using remote sensing data [3]. According to this hypothesis, blooms begin when surface heat fluxes weaken, mixing shifts from primarily convectively-driven to primarily wind-driven, and the depth of active mixing in the upper ocean consequently decreases. Here, we compare the importance of the barriers to mixing presented by submesoscale instabilities with the decreases in mixing depth caused by changes in surface forcing in driving the initiation of the spring bloom prior to the onset of surface heating. To make this comparison, we use a Lagrangian framework to track the light history of particles seeded in a high-resolution numerical model that we initialize with various surface forcing scenarios, and with and without lateral density gradients. Because the model parameterizes convection with convective adjustment, we present two methodologies to account for turbulent mixing processes that utilize observations of turbulent vertical mixing from a Lagrangian float. We present conclusions on whether and how submesoscale processes affect bloom initiation under varied surface forcing conditions in the context of whether the timing of the subpolar phytoplankton bloom can be thought of as a basin-scale or submesoscale phenomenon. [1] A. Mahadevan et al.. Science 337, 6090 (2012). [2] Brody, S.R. and Lozier, M.S. (under review, ICES J. Mar. Sci) [3] Brody, S.R. and Lozier, M.S. Geophys. Res. Lett. 41, (2014).

Density Anomalies in the Mantle and the Gravitational Core-Mantle Interaction

NASA Technical Reports Server (NTRS)

Kuang, Weijia; Liu, Lanbo

2003-01-01

Seismic studies suggest that the bulk of the mantle is heterogeneous, with density variations in depth as well as in horizontal directions (latitude and longitude). This density variation produces a three- dimensional gravity field throughout the Earth. On the other hand, the core density also varies in both time and space, due to convective core flow. Consequently, the fluid outer core and the solid mantle interact gravitationally due to the mass anomalies in both regions. This gravitational core-mantle interaction could play a significant role in exchange of angular momentum between the core and the mantle, and thus the change in Earth's rotation on time scales of decades and longer. Aiming at estimating the significance of the gravitational core-mantle interaction on Earth's rotation variation, we introduce in our MoSST core dynamics model a heterogeneous mantle, with a density distribution derived from seismic results. In this model, the core convection is driven by the buoyancy forces. And the density variation is determined dynamically with the convection. Numerical simulation is carried out with different parameter values, intending to extrapolate numerical results for geophysical implications.

Simulations of material mixing in laser-driven reshock experiments

NASA Astrophysics Data System (ADS)

Haines, Brian M.; Grinstein, Fernando F.; Welser-Sherrill, Leslie; Fincke, James R.

2013-02-01

We perform simulations of a laser-driven reshock experiment [Welser-Sherrill et al., High Energy Density Phys. (unpublished)] in the strong-shock high energy-density regime to better understand material mixing driven by the Richtmyer-Meshkov instability. Validation of the simulations is based on direct comparison of simulation and radiographic data. Simulations are also compared with published direct numerical simulation and the theory of homogeneous isotropic turbulence. Despite the fact that the flow is neither homogeneous, isotropic nor fully turbulent, there are local regions in which the flow demonstrates characteristics of homogeneous isotropic turbulence. We identify and isolate these regions by the presence of high levels of turbulent kinetic energy (TKE) and vorticity. After reshock, our analysis shows characteristics consistent with those of incompressible isotropic turbulence. Self-similarity and effective Reynolds number assessments suggest that the results are reasonably converged at the finest resolution. Our results show that in shock-driven transitional flows, turbulent features such as self-similarity and isotropy only fully develop once de-correlation, characteristic vorticity distributions, and integrated TKE, have decayed significantly. Finally, we use three-dimensional simulation results to test the performance of two-dimensional Reynolds-averaged Navier-Stokes simulations. In this context, we also test a presumed probability density function turbulent mixing model extensively used in combustion applications.

Stability of Electrodeposition at Solid-Solid Interfaces and Implications for Metal Anodes

NASA Astrophysics Data System (ADS)

Ahmad, Zeeshan; Viswanathan, Venkatasubramanian

2017-08-01

We generalize the conditions for stable electrodeposition at isotropic solid-solid interfaces using a kinetic model which incorporates the effects of stresses and surface tension at the interface. We develop a stability diagram that shows two regimes of stability: a previously known pressure-driven mechanism and a new density-driven stability mechanism that is governed by the relative density of metal in the two phases. We show that inorganic solids and solid polymers generally do not lead to stable electrodeposition, and provide design guidelines for achieving stable electrodeposition.

Homogeneous internal wave turbulence driven by tidal flows

NASA Astrophysics Data System (ADS)

Le Reun, Thomas; Favier, Benjamin; Le Bars, Michael; Erc Fludyco Team

2017-11-01

We propose a novel investigation of the stability of strongly stratified planetary fluid layers undergoing periodic tidal distortion in the limit where rotational effects are negligible compared to buoyancy. With the help of a local model focusing on a small fluid area compared to the global layer, we find that periodic tidal distortion drives a parametric subharmonic resonance of internal. This instability saturates into an homogeneous internal wave turbulence pervading the whole fluid interior: the energy is injected in the unstable waves which then feed a succession of triadic resonances also generating small spatial scales. As the timescale separation between the forcing and Brunt-Väisälä is increased, the temporal spectrum of this turbulence displays a -2 power law reminiscent of the Garrett and Munk spectrum measured in the oceans (Garett & Munk 1979). Moreover, in this state consisting of a superposition of waves in weak non-linear interaction, the mixing efficiency is increased compared to classical, Kolmogorov-like stratified turbulence. This study is of wide interest in geophysical fluid dynamics ranging from oceanic turbulence and tidal heating in icy satellites to dynamo action in partially stratified planetary cores as it could be the case in the Earth. We acknowledge support from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement No. 681835-FLUDYCO-ERC-2015-CoG).

Stratified Simulations of Collisionless Accretion Disks

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hirabayashi, Kota; Hoshino, Masahiro, E-mail: hirabayashi-k@eps.s.u-tokyo.ac.jp

This paper presents a series of stratified-shearing-box simulations of collisionless accretion disks in the recently developed framework of kinetic magnetohydrodynamics (MHD), which can handle finite non-gyrotropy of a pressure tensor. Although a fully kinetic simulation predicted a more efficient angular-momentum transport in collisionless disks than in the standard MHD regime, the enhanced transport has not been observed in past kinetic-MHD approaches to gyrotropic pressure anisotropy. For the purpose of investigating this missing link between the fully kinetic and MHD treatments, this paper explores the role of non-gyrotropic pressure and makes the first attempt to incorporate certain collisionless effects into disk-scale,more » stratified disk simulations. When the timescale of gyrotropization was longer than, or comparable to, the disk-rotation frequency of the orbit, we found that the finite non-gyrotropy selectively remaining in the vicinity of current sheets contributes to suppressing magnetic reconnection in the shearing-box system. This leads to increases both in the saturated amplitude of the MHD turbulence driven by magnetorotational instabilities and in the resultant efficiency of angular-momentum transport. Our results seem to favor the fast advection of magnetic fields toward the rotation axis of a central object, which is required to launch an ultra-relativistic jet from a black hole accretion system in, for example, a magnetically arrested disk state.« less

Cognitive Frames of Reference and Strategic Thinking

DTIC Science & Technology

1991-04-05

Elliot Jaques and T. 0. Jacobs, whose Stratified Systems Theory (SST) links leadership requirements to organizational functions. SST emphasizes the...reverse if necessary and identify by block number) Using Stratified Systems Theory and the research on expertise as a conceptual framework, this study...Stratified Systems Theory and the research on expertise as a conceptual framework, this study explored the differences in the structure and content of the

Floating potential in electronegative plasmas for non-zero ion temperatures

NASA Astrophysics Data System (ADS)

Regodón, Guillermo Fernando; Fernández Palop, José Ignacio; Tejero-del-Caz, Antonio; Díaz-Cabrera, Juan Manuel; Carmona-Cabezas, Rafael; Ballesteros, Jerónimo

2018-02-01

The floating potential of a Langmuir probe immersed in an electronegative plasma is studied theoretically under the assumption of radial positive ion fluid movement for non-zero positive ion temperature: both cylindrical and spherical geometries are studied. The model is solvable exactly. The special characteristics of the electronegative pre-sheath are found and the influence of the stratified electronegative pre-sheath is shown to be very small in practical applications. It is suggested that the use of the floating potential in the measurement of negative ions population density is convenient, in view of the numerical results obtained. The differences between the two radial geometries, which become very important for small probe radii of the order of magnitude of the Debye length, are studied.

Directional power absorption in helicon plasma sources excited by a half-helix antenna

NASA Astrophysics Data System (ADS)

Afsharmanesh, Mohsen; Habibi, Morteza

2017-10-01

This paper deals with the investigation of the power absorption in helicon plasma excited through a half-helix antenna driven at 13.56 {{MHz}}. The simulations were carried out by means of a code, HELIC. They were carried out by taking into account different inhomogeneous radial density profiles and for a wide range of plasma densities, from {10}11 {{{cm}}}-3 to {10}13 {{{cm}}}-3. The magnetic field was 200, 400, 600 and 1000 {{G}}. A three-parameter function was used for generating various density profiles with different volume gradients, edge gradients and density widths. The density profile had a large effect on the efficient Trivelpiece-Gould (TG) and helicon mode excitation and antenna coupling to the plasma. The fraction of power deposition via the TG mode was extremely dependent on the plasma density near the plasma boundary. Interestingly, the obtained efficient parallel helicon wavelength was close to the anticipated value for Gaussian radial density profile. Power deposition was considerably asymmetric when the \\tfrac{n}{{B}0} ratio was more than a specific value for a determined density width. The longitudinal power absorption was symmetric at approximately {n}0={10}11 {{{cm}}}-3, irrespective of the magnetic field supposed. The asymmetry became more pronounced when the plasma density was {10}12 {{{cm}}}-3. The ratio of density width to the magnetic field was an important parameter in the power coupling. At high magnetic fields, the maximum of the power absorption was reached at higher plasma density widths. There was at least one combination of the plasma density, magnetic field and density width for which the RF power deposition at both side of the tube reached its maximum value.

Measuring mixing efficiency in experiments of strongly stratified turbulence

NASA Astrophysics Data System (ADS)

Augier, P.; Campagne, A.; Valran, T.; Calpe Linares, M.; Mohanan, A. V.; Micard, D.; Viboud, S.; Segalini, A.; Mordant, N.; Sommeria, J.; Lindborg, E.

2017-12-01

Oceanic and atmospheric models need better parameterization of the mixing efficiency. Therefore, we need to measure this quantity for flows representative of geophysical flows, both in terms of types of flows (with vortices and/or waves) and of dynamical regimes. In order to reach sufficiently large Reynolds number for strongly stratified flows, experiments for which salt is used to produce the stratification have to be carried out in a large rotating platform of at least 10-meter diameter.We present new experiments done in summer 2017 to study experimentally strongly stratified turbulence and mixing efficiency in the Coriolis platform. The flow is forced by a slow periodic movement of an array of large vertical or horizontal cylinders. The velocity field is measured by 3D-2C scanned horizontal particles image velocimetry (PIV) and 2D vertical PIV. Six density-temperature probes are used to measure vertical and horizontal profiles and signals at fixed positions.We will show how we rely heavily on open-science methods for this study. Our new results on the mixing efficiency will be presented and discussed in terms of mixing parameterization.

Current quantization and fractal hierarchy in a driven repulsive lattice gas.

PubMed

Rotondo, Pietro; Sellerio, Alessandro Luigi; Glorioso, Pietro; Caracciolo, Sergio; Cosentino Lagomarsino, Marco; Gherardi, Marco

2017-11-01

Driven lattice gases are widely regarded as the paradigm of collective phenomena out of equilibrium. While such models are usually studied with nearest-neighbor interactions, many empirical driven systems are dominated by slowly decaying interactions such as dipole-dipole and Van der Waals forces. Motivated by this gap, we study the nonequilibrium stationary state of a driven lattice gas with slow-decayed repulsive interactions at zero temperature. By numerical and analytical calculations of the particle current as a function of the density and of the driving field, we identify (i) an abrupt breakdown transition between insulating and conducting states, (ii) current quantization into discrete phases where a finite current flows with infinite differential resistivity, and (iii) a fractal hierarchy of excitations, related to the Farey sequences of number theory. We argue that the origin of these effects is the competition between scales, which also causes the counterintuitive phenomenon that crystalline states can melt by increasing the density.

Guiding of relativistic electron beams in dense matter by laser-driven magnetostatic fields.

PubMed

Bailly-Grandvaux, M; Santos, J J; Bellei, C; Forestier-Colleoni, P; Fujioka, S; Giuffrida, L; Honrubia, J J; Batani, D; Bouillaud, R; Chevrot, M; Cross, J E; Crowston, R; Dorard, S; Dubois, J-L; Ehret, M; Gregori, G; Hulin, S; Kojima, S; Loyez, E; Marquès, J-R; Morace, A; Nicolaï, Ph; Roth, M; Sakata, S; Schaumann, G; Serres, F; Servel, J; Tikhonchuk, V T; Woolsey, N; Zhang, Z

2018-01-09

Intense lasers interacting with dense targets accelerate relativistic electron beams, which transport part of the laser energy into the target depth. However, the overall laser-to-target energy coupling efficiency is impaired by the large divergence of the electron beam, intrinsic to the laser-plasma interaction. Here we demonstrate that an efficient guiding of MeV electrons with about 30 MA current in solid matter is obtained by imposing a laser-driven longitudinal magnetostatic field of 600 T. In the magnetized conditions the transported energy density and the peak background electron temperature at the 60-μm-thick target's rear surface rise by about a factor of five, as unfolded from benchmarked simulations. Such an improvement of energy-density flux through dense matter paves the ground for advances in laser-driven intense sources of energetic particles and radiation, driving matter to extreme temperatures, reaching states relevant for planetary or stellar science as yet inaccessible at the laboratory scale and achieving high-gain laser-driven thermonuclear fusion.

Current quantization and fractal hierarchy in a driven repulsive lattice gas

NASA Astrophysics Data System (ADS)

Rotondo, Pietro; Sellerio, Alessandro Luigi; Glorioso, Pietro; Caracciolo, Sergio; Cosentino Lagomarsino, Marco; Gherardi, Marco

2017-11-01

Driven lattice gases are widely regarded as the paradigm of collective phenomena out of equilibrium. While such models are usually studied with nearest-neighbor interactions, many empirical driven systems are dominated by slowly decaying interactions such as dipole-dipole and Van der Waals forces. Motivated by this gap, we study the nonequilibrium stationary state of a driven lattice gas with slow-decayed repulsive interactions at zero temperature. By numerical and analytical calculations of the particle current as a function of the density and of the driving field, we identify (i) an abrupt breakdown transition between insulating and conducting states, (ii) current quantization into discrete phases where a finite current flows with infinite differential resistivity, and (iii) a fractal hierarchy of excitations, related to the Farey sequences of number theory. We argue that the origin of these effects is the competition between scales, which also causes the counterintuitive phenomenon that crystalline states can melt by increasing the density.

Laser-driven clockwise molecular rotation for a transient spinning waveplate.

PubMed

York, Andrew G

2009-08-03

Our simulations show a copropagating pair of laser pulses polarized in two different directions can selectively excite clockwise or counterclockwise molecular rotation in a gas of linear molecules. The resulting birefringence of the gas rotates on a femtosecond timescale and shows a periodic revival structure. The total duration of the pulse pair can be subpicosecond, allowing molecular alignment at the high densities and temperatures necessary to create a transient spinning waveplate.

Stochastically generating tree diameter lists to populate forest stands based on the linkage variables forest type and stand age

Treesearch

Bernard R. Parresol; F. Thomas Lloyd

2003-01-01

Forest inventory data were used to develop a standage-driven, stochastic predictor of unit-area, frequency weighted lists of breast high tree diameters (DBH). The average of mean statistics from 40 simulation prediction sets of an independent 78-plot validation dataset differed from the observed validation means by 0.5 cm for DBH, and by 12 trees/h for density. The 40...

Adjusting for multiple prognostic factors in the analysis of randomised trials

PubMed Central

2013-01-01

Background When multiple prognostic factors are adjusted for in the analysis of a randomised trial, it is unclear (1) whether it is necessary to account for each of the strata, formed by all combinations of the prognostic factors (stratified analysis), when randomisation has been balanced within each stratum (stratified randomisation), or whether adjusting for the main effects alone will suffice, and (2) the best method of adjustment in terms of type I error rate and power, irrespective of the randomisation method. Methods We used simulation to (1) determine if a stratified analysis is necessary after stratified randomisation, and (2) to compare different methods of adjustment in terms of power and type I error rate. We considered the following methods of analysis: adjusting for covariates in a regression model, adjusting for each stratum using either fixed or random effects, and Mantel-Haenszel or a stratified Cox model depending on outcome. Results Stratified analysis is required after stratified randomisation to maintain correct type I error rates when (a) there are strong interactions between prognostic factors, and (b) there are approximately equal number of patients in each stratum. However, simulations based on real trial data found that type I error rates were unaffected by the method of analysis (stratified vs unstratified), indicating these conditions were not met in real datasets. Comparison of different analysis methods found that with small sample sizes and a binary or time-to-event outcome, most analysis methods lead to either inflated type I error rates or a reduction in power; the lone exception was a stratified analysis using random effects for strata, which gave nominal type I error rates and adequate power. Conclusions It is unlikely that a stratified analysis is necessary after stratified randomisation except in extreme scenarios. Therefore, the method of analysis (accounting for the strata, or adjusting only for the covariates) will not generally need to depend on the method of randomisation used. Most methods of analysis work well with large sample sizes, however treating strata as random effects should be the analysis method of choice with binary or time-to-event outcomes and a small sample size. PMID:23898993

Segregation of large granules from close-packed cluster of small granules due to buoyancy.

PubMed

Yang, Xian-qing; Zhou, Kun; Qiu, Kang; Zhao, Yue-min

2006-03-01

Segregation of large granules in a vibrofluidized granular bed with inhomogeneous granular number density distribution is studied by an event-driven algorithm. Simulation results show that the mean vertical position of large granules decreases with the increase of the density ration of the large granules to the small ones. This conclusion is consistent with the explanation that the net pressure due to the small surrounding particle impacts balances the large granular weight, and indict that the upward movement of the large granules is driven by the buoyancy. The values of temperature, density, and pressure of the systems are also computed by changing the conditions such as heating temperature on the bottom and restitution coefficient of particles. These results indicate that the segregation of large granules also happen in the systems with density inversion or even close-packed cluster of particles floating on a low-density fluid, due to the buoyancy. An equation of state is proposed to explain the buoyancy.

Experimental studies of interactions between Alfv'en waves and striated density depletions in the LAPD

NASA Astrophysics Data System (ADS)

Auerbach, D. W.; Carter, T. A.; Vincena, S.

2008-11-01

Satellite measurements in the earth's magnetosphere have associated Alfv'en frequency fluctuations with density depletions striated along the geomagnetic field. This poster presents laboratory studies in the LADP experiment at UCLA modeling this phenomena. Density depletions are pre-formed in the plasma column by selectively blocking a portion of the drive beam, and Alfv'en waves are driven in the cavity by means of an inserted antenna. Relevant experimental parameters include an ion cyclotron radius around a mm, alfven parallel wavelength several meters, electron inertial length around 6 mm, and electron thermal speeds about a third of the alfv'en speed. We report here on modifications to the wave propagation due to the density depletion. We also report on the details of the interactions between the driven wave and the secondary drift-alfv'en wave instabilities that arise on the density boundary, including wave-wave interactions and possible turbulent broadening effects on the main wave.

DENSITY-MAGNETIC FIELD CORRELATION IN MAGNETOHYDRODYNAMIC TURBULENCE DRIVEN BY DIFFERENT DRIVING SCHEMES WITH DIFFERENT CORRELATION TIMES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yoon, Heesun; Cho, Jungyeon; Kim, Jongsoo, E-mail: hsyoon@cnu.ac.kr, E-mail: jcho@cnu.ac.kr, E-mail: jskim@kasi.re.kr

Turbulent motions naturally produce density and magnetic-field fluctuations. Correlation between the two fluctuations is important for interpretation of observations, such as observations of the rotation measure (RM). In this paper, we study the effect of driving schemes on the density-magnetic-field correlation. In particular, we numerically investigate how the correlation time of driving affects the correlation between density and magnetic field. We perform compressible magnetohydrodynamic turbulence simulations at different sonic Mach numbers ( M {sub s} ), using two different driving schemes—a finite-correlated driving and a delta-correlated driving. In the former, the forcing vectors change continuously with a correlation time comparablemore » to the large-eddy turnover time. In the latter, the direction (and amplitude) of driving changes in a very short timescale. The finite-correlated driving results in strong anti-correlation between two fields when the sonic and the Alfvénic Mach numbers are similar to unity (i.e., when M {sub s} ∼ 1 and M {sub A} ∼ 1, respectively). However, the anti-correlation becomes weaker and approaches zero for higher values of M {sub s} or M {sub A}. The delta-correlated driving produces virtually no correlation between two fields when M {sub s} ∼ 1 and M {sub A} ∼ 1, and produces more and more positive correlations as M {sub s} or M {sub A} increases. We conjecture that two competing effects, tendency for achieving balance between the gas and the magnetic pressure and simultaneous compression of fluid and magnetic field, determine the correlation behavior. We also investigate how different driving schemes affect the Probability Density Function of three-dimensional density, dispersion measure, and RM.« less

Idealised large-eddy-simulation of thermally driven flows over an isolated mountain range with multiple ridges

NASA Astrophysics Data System (ADS)

Lang, Moritz N.; Gohm, Alexander; Wagner, Johannes S.; Leukauf, Daniel; Posch, Christian

2014-05-01

Two dimensional idealised large-eddy-simulations are performed using the WRF model to investigate thermally driven flows during the daytime over complex terrain. Both the upslope flows and the temporal evolution of the boundary layer structure are studied with a constant surface heat flux forcing of 150 W m-2. In order to distinguish between different heating processes the flow is Reynold decomposed into its mean and turbulent part. The heating processes associated with the mean flow are a cooling through cold-air advection along the slopes and subsidence warming within the valleys. The turbulent component causes bottom-up heating near the ground leading to a convective boundary layer (CBL) inside the valleys. Overshooting potentially colder thermals cool the stably stratified valley atmosphere above the CBL. Compared to recent investigations (Schmidli 2013, J. Atmos. Sci., Vol. 70, No. 12: pp. 4041-4066; Wagner et al. 2014, manuscript submitted to Mon. Wea. Rev.), which used an idealised topography with two parallel mountain crests separated by a straight valley, this project focuses on multiple, periodic ridges and valleys within an isolated mountain range. The impact of different numbers of ridges on the flow structure is compared with the sinusoidal envelope-topography. The present simulations show an interaction between the smaller-scale upslope winds within the different valleys and the large-scale flow of the superimposed mountain-plain wind circulation. Despite a smaller boundary layer air volume in the envelope case compared to the multiple ridges case the volume averaged heating rates are comparable. The reason is a stronger advection-induced cooling along the slopes and a weaker warming through subsidence at the envelope-topography compared to the mountain range with multiple ridges.

Urban-rural differences in BMI in low- and middle-income countries: the role of socioeconomic status123

PubMed Central

Neuman, Melissa; Kawachi, Ichiro; Gortmaker, Steven; Subramanian, S V

2013-01-01

Background: Urbanization is often cited as a main cause of increasing BMIs in low- and middle-income countries (LMICs), and urban residents in LMICs tend to have higher BMIs than do rural residents. However, urban-rural differences may be driven by differences in socioeconomic status (SES). Objective: Using nationally representative data collected at 2 time points in 38 LMICs, we assessed the association between urban residence and BMI before and after adjustment for measures of individual- and household-level SES. Design: We conducted a cross-sectional analysis of nationally representative samples of 678,471 nonpregnant women aged 15–49 y, with 225,312 women in the earlier round of surveys conducted between 1991 and 2004 and 453,159 women in the later round conducted between 1998 and 2010. We used linear and ordered multinomial analysis with a country fixed effect to obtain a pooled estimate and a country-stratified analysis. Results: We found that mean BMI (kg/m2) in less-developed countries was generally higher within urban areas (excess BMI associated with urban residence before wealth index adjustment: 1.55; 95% CI: 1.52, 1.57). However, the urban association was attenuated after SES was accounted for (association after adjustment: 0.44; 95% CI: 0.41, 0.47). Individual- and household-level SES measures were independently and positively associated with BMI. Conclusion: The association between urban residence and obesity in LMICs is driven largely by higher individual- and community-level SES in urban areas, which suggests that urban residence alone may not cause increased body weight in developing countries. PMID:23283503

A comparison of respondent-driven and venue-based sampling of female sex workers in Liuzhou, China

PubMed Central

Weir, Sharon S; Merli, M Giovanna; Li, Jing; Gandhi, Anisha D; Neely, William W; Edwards, Jessie K; Suchindran, Chirayath M; Henderson, Gail E; Chen, Xiang-Sheng

2012-01-01

Objectives To compare two methods for sampling female sex workers (FSWs) for bio-behavioural surveillance. We compared the populations of sex workers recruited by the venue-based Priorities for Local AIDS Control Efforts (PLACE) method and a concurrently implemented network-based sampling method, respondent-driven sampling (RDS), in Liuzhou, China. Methods For the PLACE protocol, all female workers at a stratified random sample of venues identified as places where people meet new sexual partners were interviewed and tested for syphilis. Female workers who reported sex work in the past 4 weeks were categorised as FSWs. RDS used peer recruitment and chain referral to obtain a sample of FSWs. Data were collected between October 2009 and January 2010. We compared the socio-demographic characteristics and the percentage with a positive syphilis test of FSWs recruited by PLACE and RDS. Results The prevalence of a positive syphilis test was 24% among FSWs recruited by PLACE and 8.5% among those recruited by RDS and tested (prevalence ratio 3.3; 95% CI 1.5 to 7.2). Socio-demographic characteristics (age, residence and monthly income) also varied by sampling method. PLACE recruited fewer FSWs than RDS (161 vs 583), was more labour-intensive and had difficulty gaining access to some venues. RDS was more likely to recruit from areas near the RDS office and from large low prevalence entertainment venues. Conclusions Surveillance protocols using different sampling methods can obtain different estimates of prevalence and population characteristics. Venue-based and network-based methods each have strengths and limitations reflecting differences in design and assumptions. We recommend that more research be conducted on measuring bias in bio-behavioural surveillance. PMID:23172350

Two-dimensional numerical study of two counter-propagating helium plasma jets in air at atmospheric pressure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yan, Wen; Sang, Chaofeng; Wang, Dezhen, E-mail: wangdez@dlut.edu.cn

In this paper, a computational study of two counter-propagating helium plasma jets in ambient air is presented. A two-dimensional fluid model is applied to investigate the physical processes of the two plasma jets interaction (PJI) driven by equal and unequal voltages, respectively. In all studied cases, the PJI results in a decrease of both plasma bullets propagation velocity. When the two plasma jets are driven by equal voltages, they never merge but rather approach each other around the middle of the gas gap at a minimum approach distance, and the minimal distance decreases with the increase of both the appliedmore » voltages and initial electron density, but increases with the increase of the relative permittivity. When the two plasma jets are driven by unequal voltages, we observe the two plasma jets will merge at the position away from the middle of the gas gap. The effect of applied voltage difference on the PJI is also studied.« less

Role of magnetic fluctuations in mode selection of magnetically driven instabilities

NASA Astrophysics Data System (ADS)

Dan, Jia-Kun; Ren, Xiao-Dong; Huang, Xian-Bin; Ouyang, Kai; Chen, Guang-Hua

2014-12-01

The influences of magnetic fluctuations on quasiperiodic structure formation and fundamental wavelength selection of the instability have been studied using two 25-μm-diameter tungsten wires on a 100 ns rise time, 220 kA pulsed power facility. Two different load configurations were adopted to make end surfaces of electrodes approximately satisfy reflecting and absorbing boundary conditions, respectively. The experimental results that the fundamental wavelength in the case of absorbing boundary condition is about one half of that in the case of reflecting boundary condition have demonstrated that magnetic fluctuations appear to play a key role in mode selection of magnetically driven instabilities. The dominant wavelength should be proportional to magnetic field and inversely proportional to square root of mass density, provided that the magnetosonic wave propagating perpendicular to magnetic fields provides a leading candidate for magnetic fluctuations. Therefore, magnetic fluctuation is one of the three key perturbations, along with surface contaminants and surface roughness, that seeds magnetically driven instabilities.

Repetition rates in heavy ion beam driven fusion reactors

NASA Astrophysics Data System (ADS)

Peterson, Robert R.

1986-01-01

The limits on the cavity gas density required for beam propagation and condensation times for material vaporized by target explosions can determine the maximum repetition rate of Heavy Ion Beam (HIB) driven fusion reactors. If the ions are ballistically focused onto the target, the cavity gas must have a density below roughly 10-4 torr (3×1012 cm-3) at the time of propagation; other propagation schemes may allow densities as high as 1 torr or more. In some reactor designs, several kilograms of material may be vaporized off of the target chamber walls by the target generated x-rays, raising the average density in the cavity to 100 tor or more. A one-dimensional combined radiation hydrodynamics and vaporization and condensation computer code has been used to simulate the behavior of the vaporized material in the target chambers of HIB fusion reactors.

Comprehensive numerical methodology for direct numerical simulations of compressible Rayleigh-Taylor instability

DOE Office of Scientific and Technical Information (OSTI.GOV)

Reckinger, Scott James; Livescu, Daniel; Vasilyev, Oleg V.

A comprehensive numerical methodology has been developed that handles the challenges introduced by considering the compressive nature of Rayleigh-Taylor instability (RTI) systems, which include sharp interfacial density gradients on strongly stratified background states, acoustic wave generation and removal at computational boundaries, and stratification-dependent vorticity production. The computational framework is used to simulate two-dimensional single-mode RTI to extreme late-times for a wide range of flow compressibility and variable density effects. The results show that flow compressibility acts to reduce the growth of RTI for low Atwood numbers, as predicted from linear stability analysis.

Double-diffusive convection and baroclinic instability in a differentially heated and initially stratified rotating system: the barostrat instability

NASA Astrophysics Data System (ADS)

Vincze, Miklos; Borcia, Ion; Harlander, Uwe; Le Gal, Patrice

2016-12-01

A water-filled differentially heated rotating annulus with initially prepared stable vertical salinity profiles is studied in the laboratory. Based on two-dimensional horizontal particle image velocimetry data and infrared camera visualizations, we describe the appearance and the characteristics of the baroclinic instability in this original configuration. First, we show that when the salinity profile is linear and confined between two non-stratified layers at top and bottom, only two separate shallow fluid layers can be destabilized. These unstable layers appear nearby the top and the bottom of the tank with a stratified motionless zone between them. This laboratory arrangement is thus particularly interesting to model geophysical or astrophysical situations where stratified regions are often juxtaposed to convective ones. Then, for more general but stable initial density profiles, statistical measures are introduced to quantify the extent of the baroclinic instability at given depths and to analyze the connections between this depth-dependence and the vertical salinity profiles. We find that, although the presence of stable stratification generally hinders full-depth overturning, double-diffusive convection can lead to development of multicellular sideways convection in shallow layers and subsequently to a multilayered baroclinic instability. Therefore we conclude that by decreasing the characteristic vertical scale of the flow, stratification may even enhance the formation of cyclonic and anticyclonic eddies (and thus, mixing) in a local sense.

Spatial distribution of marine fishes along a cross-shelf gradient containing a continuum of mangrove seagrass coral reefs off southwestern Puerto Rico

NASA Astrophysics Data System (ADS)

Aguilar-Perera, Alfonso; Appeldoorn, Richard S.

2008-01-01

Despite an extensive study of the fish community off southwestern Puerto Rico, little information is available on the fish spatial distribution along an inshore-offshore, cross-shelf gradient containing a continuum of mangrove-seagrass-coral reefs. We investigated the spatial distribution of reef-associated fish species using a stratified sampling procedure. A total of 52,138 fishes were recorded, representing 102 species belonging to 32 families. Significant differences in mean fish density were evident among strata. Mean densities at shallow fore reefs and deep fore reefs (Romero key) were significantly higher compared to the rest of strata along the gradient. Mean densities of fishes in mangroves and seagrass (Montalva Bay) were comparable to those at shallow back reefs and deep fore reefs offshore (Turrumote), but lower to those inshore (Romero); the lowest fish densities were found in mangroves and seagrass (Montalva Bay) and seagrass (Romero and Corral). At least 17 species, in 7 families, were among the most common in terms of relative abundance representing 76% of the total individuals sampled. A detrended correspondence analysis (DCA) applied to more abundant fish species showed a spatial pattern in density distribution. Three major groupings were evident corresponding to mangroves and seagrass (Montalva Bay), shallow and deep reefs (Romero), and shallow and deep reefs (Corral and Turrumote). A cluster analysis on mean fish densities of the more abundant species revealed a consistent spatial distribution according to biotope by separating the ichthyofauna associated with mangroves, seagrass and that of shallow (back and fore) reefs, and deep fore reefs.

Neighborhood economic disadvantage, violent crime, group density, and pregnancy outcomes in a diverse, urban population.

PubMed

Masi, Christopher M; Hawkley, Louise C; Piotrowski, Z Harry; Pickett, Kate E

2007-12-01

Prior research has established associations between pregnancy outcomes and specific neighborhood characteristics, including economic disadvantage, violent crime, and racial/ethnic segregation. Recently, associations have also been found between various health outcomes and group density, the degree to which an individual is a racial or ethnic majority in his or her local community. The objective of this study was to determine the extent to which census tract economic disadvantage, violent crime rate, and group density are associated with pregnancy outcomes among White, Black, and Hispanic infants in a large metropolitan setting. This cross-sectional study utilized 1990 census data, 1991 crime data, and 1991 birth certificate information for singleton live births in Chicago, Illinois. Results show substantial racial segregation in Chicago, with 35% of census tracts having more than 90% Black residents and 45% of census tracts having fewer than 10% Black residents. After stratifying by maternal race/ethnicity, we used multilevel analyses to model pregnancy outcomes as a function of individual and census tract characteristics. Among all racial/ethnic groups, violent crime rate accounted for most of the negative association between tract economic disadvantage and birth weight. Group density was also associated with birth weight but this association was stronger among Whites and Hispanics than among Blacks. Further analysis revealed that group density was more strongly associated with preterm birth while violent crime rate was more strongly associated with small for gestational age. These results suggest that group density and violent crime may impact birth weight via different mechanisms.

Modeling X-ray Absorbers in AGNs with MHD-Driven Accretion-Disk Winds

NASA Astrophysics Data System (ADS)

Fukumura, Keigo; Kazanas, D.; Shrader, C. R.; Tombesi, F.; Contopoulos, J.; Behar, E.

2013-04-01

We have proposed a systematic view of the observed X-ray absorbers, namely warm absorbers (WAs) in soft X-ray and highly-ionized ultra-fast outflows (UFOs), in the context of magnetically-driven accretion-disk wind models. While potentially complicated by variability and thermal instability in these energetic outflows, in this simplistic model we have calculated 2D kinematic field as well as density and ionization structure of the wind with density profile of 1/r corresponding to a constant column distribution per decade of ionization parameter. In particular we show semi-analytically that the inner layer of the disk-wind manifests itself as the strongly-ionized fast outflows while the outer layer is identified as the moderately-ionized absorbers. The computed characteristics of these two apparently distinct absorbers are consistent with X-ray data (i.e. a factor of ~100 difference in column and ionization parameters as well as low wind velocity vs. near-relativistic flow). With the predicted contour curves for these wind parameters one can constrain allowed regions for the presence of WAs and UFOs.The model further implies that the UFO's gas pressure is comparable to that of the observed radio jet in 3C111 suggesting that the magnetized disk-wind with density profile of 1/r is a viable agent to help sustain such a self-collimated jet at small radii.

QTAIM and Stress Tensor Characterization of Intramolecular Interactions Along Dynamics Trajectories of a Light-Driven Rotary Molecular Motor.

PubMed

Wang, Lingling; Huan, Guo; Momen, Roya; Azizi, Alireza; Xu, Tianlv; Kirk, Steven R; Filatov, Michael; Jenkins, Samantha

2017-06-29

A quantum theory of atoms in molecules (QTAIM) and stress tensor analysis was applied to analyze intramolecular interactions influencing the photoisomerization dynamics of a light-driven rotary molecular motor. For selected nonadiabatic molecular dynamics trajectories characterized by markedly different S 1 state lifetimes, the electron densities were obtained using the ensemble density functional theory method. The analysis revealed that torsional motion of the molecular motor blades from the Franck-Condon point to the S 1 energy minimum and the S 1 /S 0 conical intersection is controlled by two factors: greater numbers of intramolecular bonds before the hop-time and unusually strongly coupled bonds between the atoms of the rotor and the stator blades. This results in the effective stalling of the progress along the torsional path for an extended period of time. This finding suggests a possibility of chemical tuning of the speed of photoisomerization of molecular motors and related molecular switches by reshaping their molecular backbones to decrease or increase the degree of coupling and numbers of intramolecular bond critical points as revealed by the QTAIM/stress tensor analysis of the electron density. Additionally, the stress tensor scalar and vector analysis was found to provide new methods to follow the trajectories, and from this, new insight was gained into the behavior of the S 1 state in the vicinity of the conical intersection.

Critical dynamics of gravito-convective mixing in geological carbon sequestration

DOE PAGES

Soltanian, Mohamad Reza; Amooie, Mohammad Amin; Dai, Zhenxue; ...

2016-11-03

When CO 2 is injected in saline aquifers, dissolution causes a local increase in brine density that can cause Rayleigh-Taylor-type gravitational instabilities. Depending on the Rayleigh number, density-driven flow may mix dissolved CO 2 throughout the aquifer at fast advective time-scales through convective mixing. Heterogeneity can impact density-driven flow to different degrees. Zones with low effective vertical permeability may suppress fingering and reduce vertical spreading, while potentially increasing transverse mixing. In more complex heterogeneity, arising from the spatial organization of sedimentary facies, finger propagation is reduced in low permeability facies, but may be enhanced through more permeable facies. The connectivitymore » of facies is critical in determining the large-scale transport of CO 2-rich brine. We perform high-resolution finite element simulations of advection-diffusion transport of CO 2 with a focus on facies-based bimodal heterogeneity. Permeability fields are generated by a Markov Chain approach, which represent facies architecture by commonly observed characteristics such as volume fractions. CO 2 dissolution and phase behavior are modeled with the cubic-plus-association equation-of-state. Our results show that the organization of high-permeability facies and their connectivity control the dynamics of gravitationally unstable flow. Lastly, we discover new flow regimes in both homogeneous and heterogeneous media and present quantitative scaling relations for their temporal evolution.« less

3D Modeling of Antenna Driven Slow Waves Excited by Antennas Near the Plasma Edge

NASA Astrophysics Data System (ADS)

Smithe, David; Jenkins, Thomas

2016-10-01

Prior work with the 3D finite-difference time-domain (FDTD) plasma and sheath model used to model ICRF antennas in fusion plasmas has highlighted the possibility of slow wave excitation at the very low end of the SOL density range, and thus the prudent need for a slow-time evolution model to treat SOL density modifications due to the RF itself. At higher frequency, the DIII-D helicon antenna has much easier access to a parasitic slow wave excitation, and in this case the Faraday screen provides the dominant means of controlling the content of the launched mode, with antenna end-effects remaining a concern. In both cases, the danger is the same, with the slow-wave propagating into a lower-hybrid resonance layer a short distance ( cm) away from the antenna, which would parasitically absorb power, transferring energy to the SOL edge plasma, primarily through electron-neutral collisions. We will present 3D modeling of antennas at both ICRF and helicon frequencies. We've added a slow-time evolution capability for the SOL plasma density to include ponderomotive force driven rarefaction from the strong fields in the vicinity of the antenna, and show initial application to NSTX antenna geometry and plasma configurations. The model is based on a Scalar Ponderomotive Potential method, using self-consistently computed local field amplitudes from the 3D simulation.

Particle dispersion in a stably stratified channel flow

NASA Astrophysics Data System (ADS)

Pasquero, C.; Armenio, V.

2003-04-01

The motion of particles in a stably stratified channel flow is relevant in geophysic and environmental applications. In the present research this problem has been studied numerically using a mixed Lagrangian-Eulerian technique (Lagrangian motion of an ensemble of particles in an Eulerian field) by means of large eddy simulation. A stratified channel flows can be decomposed into a buoyancy affected region, with a strong turbulent activity, close to the walls, and into a buoyancy dominated region, where turbulence is strongly inhibited, in the center of the channel. For strong stratifications, counter gradient heat fluxes steepen the density gradient moving hot fluid up and cold fluid down. The stratification in the central region of the channel becomes extremely stable. However, the vertical turbulent energy, defined as the difference between the total vertical kinetic energy and its temporal average, is very strong. Particle statistics have shown that this can be related to the presence of high frequency internal waves, that do not contribute to dispersion because of their highly coherent behavior. Vertical stratification is shown to reduce or increase the decorrelation time for vertical motion, depending on the Richardson number. When stratification is increased there are two competing effects: Structures have a smaller vertical scale (acting to reduce the decorrelation time) and vertical velocities are smaller (acting to increase the decorrelation time, since particles stay for a longer time into a given structure in the flow). It has been shown that for low stratification the first mechanism dominates, while for large stratification the second effect is more important. The research is in progress and results for both fluid and inertial particles will be presented at the conference.

Interaction of vortex ring with a stratified finite thickness interface

NASA Astrophysics Data System (ADS)

Advaith, S.; Manu, K. V.; Tinaikar, Aashay; Chetia, Utpal Kumar; Basu, Saptarshi

2017-09-01

This work experimentally investigates the dynamics of interaction between a propagating vortex ring and density stratified interface of finite thickness. The flow evolution has been quantified using a high speed shadowgraph technique and particle image velocimetry. The spatial and temporal behaviours of the vortex in the near and far field of the interface and the plume structure formed due to buoyancy are investigated systematically by varying the vortex strength (Reynolds number, Re) and the degree of stratification (Atwood number, At). Maximum penetration length (Lpmax) of the vortex ring through the interface is measured over a range of Reynolds (1350 ≤ Re ≤ 4600) and Richardson (0.1 ≤ Ri ≤ 4) numbers. It is found that for low Froude number values, the maximum penetration length varies linearly with the Froude number as in the study of Orlandi et al. ["Vortex rings descending in a stratified fluid," Phys. Fluids 10, 2819-2827 (1998)]. However, for high Reynolds and Richardson numbers (Ri), anomalous behaviour in maximum penetration is observed. The Lpmax value is used to characterize the vortex-interface interactions into non-penetrative, partially-penetrative, and extensively penetrative regimes. Flow visualization revealed the occurrence of short-wavelength instability of a plume structure, particularly in a partially penetrative regime. Fluid motion exhibits chaotic behaviour in an extensively penetrative regime. Detailed analyses of plume structure propagation are performed by measuring the plume length and plume rise. Appropriate scaling for the plume length and plume rise is derived, which allows universal collapse of the data for different flow conditions. Some information concerning the instability of the plume structure and decay of the vortex ring is obtained using proper orthogonal decomposition.

Lagrangian clustering detection of internal wave boluses

NASA Astrophysics Data System (ADS)

Allshouse, M.; Salvador Vieira, G.; Swinney, H. L.

2016-02-01

The shoaling of internal waves on a continental slope or shelf produces boluses that travel up the slope with the wave. The boluses are regions of trapped fluid that are transported along with the wave, unlike fluid in the bulk that is temporarily pertubed by a passing wave. Boluses have been observed to transport oxygen-depleted water and induce rapid changes in temperature (Walter et al, JGR, 2012), both of which have potential ramifications for marine biology. Several previous studies have investigated boluses in systems with two layers of different density (e.g., Helfrich, JFM, 1992, and Sutherland et al., JGR, 2013). We conduct laboratory and computational studies of bolus generation and material transport in continuously stratified fluids with a pycnocline, as in the oceans. Our laboratory experiments in a 4 m long tank are complemented by 2-dimensional direct numerical simulations of the Navier-Stokes equations. Efforts have been made to identify boluses with Eularian measures in the past, but a Lagrangian perspective is necessary to objectively identify the bolus over its lifespan. Here we use a Lagrangian based coherent structure method relying on trajectory clustering using the fuzzy c-means approach (Froyland and Padberg-Gehle, Chaos, 2015). The objective detection of a bolus enables examination of the volume, distance traveled, and increased available potential energy of a bolus, as a function of the stratification, wave properties, and the angle of the sloping topography. The decay of a bolus through turbulent mixing is investigated by locating where the Richardson number drops below ¼, where velocity shear overcomes the tendency of a stratified fluid to remain stratified. (supported by ONR MURI grant N000141110701)

Density-ratio effects on buoyancy-driven variable-density turbulent mixing

NASA Astrophysics Data System (ADS)

Aslangil, Denis; Livescu, Daniel; Banerjee, Arindam

2017-11-01

Density-ratio effects on the turbulent mixing of two incompressible, miscible fluids with different densities subject to constant acceleration are studied by means of high-resolution Direct Numerical Simulations. In a triply periodic domain, turbulence is generated by stirring in response to the differential buoyancy forces within the flow. Later, as the fluids become molecularly mixed, dissipation starts to overcome turbulence generation by bouyancy. Thus, the flow evolution includes both turbulence growth and decay, and it displays features present in the core region of the mixing layer of the Rayleigh-Taylor as well as Richtmyer-Meshkov instabilities. We extend the previous studies by investigating a broad range of density-ratio, from 1-14.4:1, corresponding to Atwood numbers of 0.05-0.87. Here, we focus on the Atwood number dependence of mixing-efficiency, that is defined based on the energy-conversion ratios from potential energy to total and turbulent kinetic energies, the decay characteristics of buoyancy-assisted variable-density homogeneous turbulence, and the effects of high density-ratios on the turbulence structure and mixing process. Authors acknowledge financial support from DOE-SSAA (DE-NA0003195) and NSF CAREER (#1453056) awards.

Perceived value in food selection when dining out: comparison of African Americans and Euro-Americans.

PubMed

Vinci, Debra M; Philipp, Steven F

2007-06-01

This descriptive study compares African Americans' and Euro-Americans' perceived value of food selection pertaining to cost, portion size, and meal satisfaction when eating away from home. A stratified sample was drawn from a southern U.S. metropolitan area (N= 1,011; 486 African American, 525 Euro-American). Analysis showed no difference between African-American and Euro-American adults by sex or how often they dined out. These two groups significantly differed across years of education, age, and answering 14 of 18 rated statements on value perceptions. African-Americans' value perceptions were influenced more by lower cost foods and larger portion sizes than those of Euro-Americans. For meal satisfaction, African Americans were more likely to agree with statements that indicate preferring foods high in energy and low in essential micronutrient density. This study supports the need for more investigation.

Spatial Heterogeneity and Imperfect Mixing in Chemical Reactions: Visualization of Density-Driven Pattern Formation

DOE PAGES

Sobel, Sabrina G.; Hastings, Harold M.; Testa, Matthew

2009-01-01

Imore » mperfect mixing is a concern in industrial processes, everyday processes (mixing paint, bread machines), and in understanding salt water-fresh water mixing in ecosystems. The effects of imperfect mixing become evident in the unstirred ferroin-catalyzed Belousov-Zhabotinsky reaction, the prototype for chemical pattern formation. Over time, waves of oxidation (high ferriin concentration, blue) propagate into a background of low ferriin concentration (red); their structure reflects in part the history of mixing in the reaction vessel. However, it may be difficult to separate mixing effects from reaction effects. We describe a simpler model system for visualizing density-driven pattern formation in an essentially unmixed chemical system: the reaction of pale yellow Fe 3 + with colorless SCN − to form the blood-red Fe ( SCN ) 2 + complex ion in aqueous solution. Careful addition of one drop of Fe ( NO 3 ) 3 to KSCN yields striped patterns after several minutes. The patterns appear reminiscent of Rayleigh-Taylor instabilities and convection rolls, arguing that pattern formation is caused by density-driven mixing.« less

Cathode fall model and current-voltage characteristics of field emission driven direct current microplasmas

DOE Office of Scientific and Technical Information (OSTI.GOV)

Venkattraman, Ayyaswamy

2013-11-15

The post-breakdown characteristics of field emission driven microplasma are studied theoretically and numerically. A cathode fall model assuming a linearly varying electric field is used to obtain equations governing the operation of steady state field emission driven microplasmas. The results obtained from the model by solving these equations are compared with particle-in-cell with Monte Carlo collisions simulation results for parameters including the plasma potential, cathode fall thickness, ion number density in the cathode fall, and current density vs voltage curves. The model shows good overall agreement with the simulations but results in slightly overpredicted values for the plasma potential andmore » the cathode fall thickness attributed to the assumed electric field profile. The current density vs voltage curves obtained show an arc region characterized by negative slope as well as an abnormal glow discharge characterized by a positive slope in gaps as small as 10 μm operating at atmospheric pressure. The model also retrieves the traditional macroscale current vs voltage theory in the absence of field emission.« less

Spatial Heterogeneity and Imperfect Mixing in Chemical Reactions: Visualization of Density-Driven Pattern Formation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sobel, Sabrina G.; Hastings, Harold M.; Testa, Matthew

Imore » mperfect mixing is a concern in industrial processes, everyday processes (mixing paint, bread machines), and in understanding salt water-fresh water mixing in ecosystems. The effects of imperfect mixing become evident in the unstirred ferroin-catalyzed Belousov-Zhabotinsky reaction, the prototype for chemical pattern formation. Over time, waves of oxidation (high ferriin concentration, blue) propagate into a background of low ferriin concentration (red); their structure reflects in part the history of mixing in the reaction vessel. However, it may be difficult to separate mixing effects from reaction effects. We describe a simpler model system for visualizing density-driven pattern formation in an essentially unmixed chemical system: the reaction of pale yellow Fe 3 + with colorless SCN − to form the blood-red Fe ( SCN ) 2 + complex ion in aqueous solution. Careful addition of one drop of Fe ( NO 3 ) 3 to KSCN yields striped patterns after several minutes. The patterns appear reminiscent of Rayleigh-Taylor instabilities and convection rolls, arguing that pattern formation is caused by density-driven mixing.« less

Toroidal Alfvénic Eigenmodes Driven by Energetic Particles with Maxwell and Slowing-down Distributions

NASA Astrophysics Data System (ADS)

Hou, Yawei; Zhu, Ping; Zou, Zhihui; Kim, Charlson C.; Hu, Zhaoqing; Wang, Zhengxiong

2016-10-01

The energetic-particle (EP) driven toroidal Alfvén eigenmodes (TAEs) in a circular-shaped large aspect ratio tokamak are studied using the hybrid kinetic-MHD model in the NIMROD code, where the EPs are advanced using the δf particle-in-cell (PIC) method and their kinetic effects are coupled to the bulk plasma through moment closures. Two initial distributions of EPs, Maxwell and slowing-down, are considered. The influence of EP parameters, including density, temperature and density gradient, on the frequency and the growth rate of TAEs are obtained and benchmarked with theory and gyrokinetic simulations for the Maxwell distribution with good agreement. When the density and temperature of EPs are above certain thresholds, the transition from TAE to energetic particle modes (EPM) occurs and the mode structure also changes. Comparisons between Maxwell and slowing-down distributions in terms of EP-driven TAEs and EPMs will also be presented and discussed. Supported by the National Magnetic Confinement Fusion Science Program of China Grant Nos. 2014GB124002 and 2015GB101004, and the Natural Science Foundation of China Grant No. 11205194.

Onset of turbulence in accelerated high-Reynolds-number flow

NASA Astrophysics Data System (ADS)

Zhou, Ye; Robey, Harry F.; Buckingham, Alfred C.

2003-05-01

A new criterion, flow drive time, is identified here as a necessary condition for transition to turbulence in accelerated, unsteady flows. Compressible, high-Reynolds-number flows initiated, for example, in shock tubes, supersonic wind tunnels with practical limitations on dimensions or reservoir capacity, and high energy density pulsed laser target vaporization experimental facilities may not provide flow duration adequate for turbulence development. In addition, for critical periods of the overall flow development, the driving background flow is often unsteady in the experiments as well as in the physical flow situations they are designed to mimic. In these situations transition to fully developed turbulence may not be realized despite achievement of flow Reynolds numbers associated with or exceeding stationary flow transitional criteria. Basically our transitional criterion and prediction procedure extends to accelerated, unsteady background flow situations the remarkably universal mixing transition criterion proposed by Dimotakis [P. E. Dimotakis, J. Fluid Mech. 409, 69 (2000)] for stationary flows. This provides a basis for the requisite space and time scaling. The emphasis here is placed on variable density flow instabilities initiated by constant acceleration Rayleigh-Taylor instability (RTI) or impulsive (shock) acceleration Richtmyer-Meshkov instability (RMI) or combinations of both. The significant influences of compressibility on these developing transitional flows are discussed with their implications on the procedural model development. A fresh perspective for predictive modeling and design of experiments for the instability growth and turbulent mixing transitional interval is provided using an analogy between the well-established buoyancy-drag model with applications of a hierarchy of single point turbulent transport closure models. Experimental comparisons with the procedural results are presented where use is made of three distinctly different types of acceleration driven instability experiments: (1) classical, relatively low speed, constant acceleration RTI experiments; (2) shock tube, shockwave driven RMI flow mixing experiments; (3) laser target vaporization RTI and RMI mixing experiments driven at very high energy density. These last named experiments are of special interest as they provide scaleable flow conditions simulating those of astrophysical magnitude such as shock-driven hydrodynamic mixing in supernova evolution research.

Internal combustion engine using premixed combustion of stratified charges

DOEpatents

Marriott, Craig D [Rochester Hills, MI; Reitz, Rolf D [Madison, WI

2003-12-30

During a combustion cycle, a first stoichiometrically lean fuel charge is injected well prior to top dead center, preferably during the intake stroke. This first fuel charge is substantially mixed with the combustion chamber air during subsequent motion of the piston towards top dead center. A subsequent fuel charge is then injected prior to top dead center to create a stratified, locally richer mixture (but still leaner than stoichiometric) within the combustion chamber. The locally rich region within the combustion chamber has sufficient fuel density to autoignite, and its self-ignition serves to activate ignition for the lean mixture existing within the remainder of the combustion chamber. Because the mixture within the combustion chamber is overall premixed and relatively lean, NO.sub.x and soot production are significantly diminished.

An Investigation of the Effects of Internal Waves on Sound Propagation in a Stratified Medium with a Sloping Bed

NASA Astrophysics Data System (ADS)

Deldar, H.; Bidokhti, A. A.; Chegini, V.

2018-01-01

Internal waves usually cause temporal and spatial changes of density and consequently affect the acoustic wave propagation in the ocean. The purpose of this study is a laboratory investigation of the effects of internal waves generated by oscillation of a cylinder in a large stratified glass tank with a sloping bed on the sound waves propagation. Results showed that sound waves are affected by internal waves that depend on the slope angle to the direction of internal wave propagation angle ratio. When the ratio is subcritical or supercritical, the acoustic signal is much reduced as compared to the case with no sloped bottom. This can be explained in terms of the internal waves energy reaching the sloped bed and their reflections.

A Data-Driven Simulation of HIV Spread Among Young Men Who Have Sex With Men: Role of Age and Race Mixing and STIs.

PubMed

Beck, Ekkehard C; Birkett, Michelle; Armbruster, Benjamin; Mustanski, Brian

2015-10-01

Young men who have sex with men (YMSM) in the United States have a high HIV incidence with substantial racial disparities that are poorly understood. We use a data-driven simulation model to understand the impact of network-level mechanisms and sexually transmitted infections on the spread of HIV among YMSM. We designed and parameterized a stochastic agent-based network simulation model using results of a longitudinal cohort study of YMSM in Chicago. Within this model, YMSM formed and dissolved partnerships over time, and partnership types were stratified by length of partnership, sex, and age of the partner. In each partnership, HIV, gonorrhea, and chlamydia could be transmitted. Counterfactual scenarios were run to examine drivers of HIV. Over a 15-year simulation, the HIV epidemic among YMSM continued to rise, with Latino/white YMSM facing a steeper increase in the HIV burden compared with black YMSM. YMSM in partnerships with older MSM, in particular black YMSM with older black MSM, were at highest risk for HIV, and 1 infection prevented with an older partner would prevent 0.8 additional infections among YMSM. Additionally, racial disparities in HIV were driven by differences in the HIV prevalence of YMSM partners. Finally, of all HIV infections among YMSM, 14.6% were attributable to NG and CT infections. Network-level mechanisms and sexually transmitted infections play a significant role in the spread of HIV and in racial disparities among YMSM. HIV prevention efforts should target YMSM across race, and interventions focusing on YMSM partnerships with older MSM might be highly effective.

A data-driven simulation of HIV spread among young men who have sex with men: the role of age and race mixing, and STIs

PubMed Central

BECK, Ekkehard C.; BIRKETT, Michelle; ARMBRUSTER, Benjamin; MUSTANSKI, Brian

2015-01-01

Objective Young men who have sex with men (YMSM) in the U.S. have a high HIV incidence with substantial racial disparities that are poorly understood. We use a data-driven simulation model to understand the impact of network-level mechanisms and STI infections on the spread of HIV among YMSM. Methods We designed and parameterized a stochastic agent-based network simulation model using results of a longitudinal cohort study of YMSM in Chicago. Within this model, YMSM formed and dissolved partnerships over time, and partnership-types were stratified by length of partnership, sex and age of the partner. In each partnership, HIV, gonorrhea and chlamydia could be transmitted. Counterfactual scenarios were run to examine drivers of HIV. Results Over a 15 year simulation, the HIV epidemic among YMSM continued to rise with Latino/White YMSM facing a steeper increase in the HIV burden compared to Black YMSM. YMSM in partnerships with older MSM, in particular Black YMSM with older Black MSM, were at highest risk for HIV and one infection prevented with an older partner would prevent 0.8 additional infections among YMSM. Additionally, racial disparities in HIV were driven by differences in the HIV prevalence of YMSM partners. Finally, of all HIV infections among YMSM, 14.6% were attributable to NG and CT infections. Conclusion Network-level mechanisms and STI infections play a significant role in the spread of HIV, and in racial disparities among YMSM. HIV prevention efforts should target YMSM across race, and interventions focusing on YMSM partnerships with older MSM might be highly effective. PMID:26102448

Transcritical flow of a stratified fluid over topography: analysis of the forced Gardner equation

NASA Astrophysics Data System (ADS)

Kamchatnov, A. M.; Kuo, Y.-H.; Lin, T.-C.; Horng, T.-L.; Gou, S.-C.; Clift, R.; El, G. A.; Grimshaw, R. H. J.

2013-12-01

Transcritical flow of a stratified fluid past a broad localised topographic obstacle is studied analytically in the framework of the forced extended Korteweg--de Vries (eKdV), or Gardner, equation. We consider both possible signs for the cubic nonlinear term in the Gardner equation corresponding to different fluid density stratification profiles. We identify the range of the input parameters: the oncoming flow speed (the Froude number) and the topographic amplitude, for which the obstacle supports a stationary localised hydraulic transition from the subcritical flow upstream to the supercritical flow downstream. Such a localised transcritical flow is resolved back into the equilibrium flow state away from the obstacle with the aid of unsteady coherent nonlinear wave structures propagating upstream and downstream. Along with the regular, cnoidal undular bores occurring in the analogous problem for the single-layer flow modeled by the forced KdV equation, the transcritical internal wave flows support a diverse family of upstream and downstream wave structures, including solibores, rarefaction waves, reversed and trigonometric undular bores, which we describe using the recent development of the nonlinear modulation theory for the (unforced) Gardner equation. The predictions of the developed analytic construction are confirmed by direct numerical simulations of the forced Gardner equation for a broad range of input parameters.

Zooplankton diel vertical migration and contribution to deep active carbon flux in the NW Mediterranean

NASA Astrophysics Data System (ADS)

Isla, Alejandro; Scharek, Renate; Latasa, Mikel

2015-03-01

The diel vertical migration (DVM) of zooplankton contributes to the biological pump transporting material from surface to deep waters. We examined the DVM of the zooplankton community in different size fractions (53-200 μm, 200-500 μm, 500-1000 μm, 1000-2000 μm and > 2000 μm) during three cruises carried out in the open NW Mediterranean Sea. We assessed their metabolic rates from empirical published relationships and estimated the active fluxes of dissolved carbon to the mesopelagic zone driven by migrant zooplankton. Within the predominantly oligotrophic Mediterranean Sea, the NW region is one of the most productive ones, with a seasonal cycle characterized by a prominent spring bloom. The study area was visited at three different phases of the seasonal cycle: during the spring bloom, the post-bloom, and strongly stratified oligotrophic conditions. We found seasonal differences in DVM, less evident during the bloom. Changes in DVM intensity were related to the composition of the zooplanktonic assemblage, which also varied between cruises. Euphausiids appeared as the most active migrants in all seasons, and their life cycle conditioned the observed pattern. Immature stages, which are unable to perform large diel vertical movements, dominated during the bloom, in contrast to the higher relative importance of migrating adults in the other two sampling periods. The amount of dissolved carbon exported was determined by the migrant zooplankton biomass, being highest during the post-bloom (2.2 mmol C respired m- 2 d- 1, and up to 3.1 mmol C exported m- 2 d- 1 when DOC release estimations are added). The active transport by diel migrants represented a substantial contribution to total carbon export to deep waters, especially under stratified oligotrophic conditions, revealing the importance of zooplankton in the biological pump operating in the study area.

75 FR 31811 - Endangered and Threatened Wildlife and Plants; Post-Delisting Monitoring Plan for Bald Eagle...

Federal Register 2010, 2011, 2012, 2013, 2014

2010-06-04

... Species Act (ESA) requires that we implement a system, in cooperation with the States, to monitor... cooperation with the States, to effectively monitor for not less than 5 years the status of all species that... stratified sampling based on density of identified bald eagle nest sites. Our Bald Eagle Monitoring Team will...

Long-distance dispersal of the gypsy moth (Lepidoptera: Lymantriidae) facilitated its initial invasion of Wisconsin

Treesearch

Patrick C. Tobin; Laura M. Blackburn

2008-01-01

Gypsy moth (Lymantria dispar L.) spread is dominated by stratified dispersal, and, although spread rates are variable in space and time, the gypsy moth has invaded Wisconsin at a consistently higher rate than in other regions. Allee effects, which act on low-density populations ahead of the moving population that contribute to gypsy moth spread, have...

Variational approach to stability boundary for the Taylor-Goldstein equation

NASA Astrophysics Data System (ADS)

Hirota, Makoto; Morrison, Philip J.

2015-11-01

Linear stability of inviscid stratified shear flow is studied by developing an efficient method for finding neutral (i.e., marginally stable) solutions of the Taylor-Goldstein equation. The classical Miles-Howard criterion states that stratified shear flow is stable if the local Richardson number JR is greater than 1/4 everywhere. In this work, the case of JR > 0 everywhere is considered by assuming strictly monotonic and smooth profiles of the ambient shear flow and density. It is shown that singular neutral modes that are embedded in the continuous spectrum can be found by solving one-parameter families of self-adjoint eigenvalue problems. The unstable ranges of wavenumber are searched for accurately and efficiently by adopting this method in a numerical algorithm. Because the problems are self-adjoint, the variational method can be applied to ascertain the existence of singular neutral modes. For certain shear flow and density profiles, linear stability can be proven by showing the non-existence of a singular neutral mode. New sufficient conditions, extensions of the Rayleigh-Fjortoft stability criterion for unstratified shear flows, are derived in this manner. This work was supported by JSPS Strategic Young Researcher Overseas Visits Program for Accelerating Brain Circulation # 55053270.

Turbulent Mixing in Gravity Currents with Transverse Shear

NASA Astrophysics Data System (ADS)

White, Brian; Helfrich, Karl; Scotti, Alberto

2010-11-01

A parallel flow with horizontal shear and horizontal density gradient undergoes an intensification of the shear by gravitational tilting and stretching, rapidly breaking down into turbulence. Such flows have the potential for substantial mixing in estuaries and the coastal ocean. We present high-resolution numerical results for the mixing efficiency of these flows, which can be viewed as gravity currents with transverse shear, and contrast them with the well-studied case of stably stratified, homogeneous turbulence (uniform vertical density and velocity gradients). For a sheared gravity current, the buoyancy flux, turbulent Reynolds stress, and dissipation are well out of equilibrium. The total kinetic energy first increases as potential energy is transferred to the gravity current, but rapidly decays once turbulence sets in. Despite the non-equilibrium character, mixing efficiencies are slightly higher but qualitatively similar to homogeneous stratified turbulence. Efficiency decreases in the highly energetic regime where the dissipation rate is large compared with viscosity and stratification, ɛ/(νN^2)>100, further declining as turbulence decays and kinetic energy dissipation dominates the buoyancy flux. In general, the mixing rate, parameterized by a turbulent eddy diffusivity, increases with the strength of the transverse shear.

Divergence in morphology, but not habitat use, despite low genetic differentiation among insular populations of the lizard Anolis lemurinus in Honduras

USGS Publications Warehouse

Logan, M.L.; Montgomery, Chad E.; Boback, Scott M.; Reed, R.N.; Campbell, J.A.

2012-01-01

Studies of recently isolated populations are useful because observed differences can often be attributed to current environmental variation. Two populations of the lizard Anolis lemurinus have been isolated on the islands of Cayo Menor and Cayo Mayor in the Cayos Cochinos Archipelago of Honduras for less than 15 000 y. We measured 12 morphometric and 10 habitat-use variables on 220 lizards across these islands in 2 y, 2008 and 2009. The goals of our study were (1) to explore patterns of sexual dimorphism, and (2) to test the hypothesis that differences in environment among islands may have driven divergence in morphology and habitat use despite genetic homogeneity among populations. Although we found no differences among sexes in habitat use, males had narrower pelvic girdles and longer toe pads on both islands. Between islands, males differed in morphology, but neither males nor females differed in habitat use. Our data suggest that either recent selection has operated differentially on males despite low genetic dill'erentiation, or that they display phenotypic plasticity in response to environmental variation. We suggest that patterns may be driven by variation in intrapopulation density or differences in predator diversity among islands.

Lateral baroclinic forcing enhances sediment transport from shallows to channel in an estuary

USGS Publications Warehouse

Lacy, Jessica R.; Gladding, Steve; Brand, Andreas; Collignon, Audric; Stacey, Mark

2014-01-01

We investigate the dynamics governing exchange of sediment between estuarine shallows and the channel based on field measurements at eight stations spanning the interface between the channel and the extensive eastern shoals of South San Francisco Bay. The study site is characterized by longitudinally homogeneous bathymetry and a straight channel, with friction more important than the Coriolis forcing. Data were collected for 3 weeks in the winter and 4 weeks in the late summer of 2009, to capture a range of hydrologic and meteorologic conditions. The greatest sediment transport from shallows to channel occurred during a pair of strong, late-summer wind events, with westerly winds exceeding 10 m/s for more than 24 h. A combination of wind-driven barotropic return flow and lateral baroclinic circulation caused the transport. The lateral density gradient was produced by differences in temperature and suspended sediment concentration (SSC). During the wind events, SSC-induced vertical density stratification limited turbulent mixing at slack tides in the shallows, increasing the potential for two-layer exchange. The temperature- and SSC-induced lateral density gradient was comparable in strength to salinity-induced gradients in South Bay produced by seasonal freshwater inflows, but shorter in duration. In the absence of a lateral density gradient, suspended sediment flux at the channel slope was directed towards the shallows, both in winter and during summer sea breeze conditions, indicating the importance of baroclinically driven exchange to supply of sediment from the shallows to the channel in South San Francisco Bay and systems with similar bathymetry.

Elucidating the electron transport in semiconductors via Monte Carlo simulations: an inquiry-driven learning path for engineering undergraduates

NASA Astrophysics Data System (ADS)

Persano Adorno, Dominique; Pizzolato, Nicola; Fazio, Claudio

2015-09-01

Within the context of higher education for science or engineering undergraduates, we present an inquiry-driven learning path aimed at developing a more meaningful conceptual understanding of the electron dynamics in semiconductors in the presence of applied electric fields. The electron transport in a nondegenerate n-type indium phosphide bulk semiconductor is modelled using a multivalley Monte Carlo approach. The main characteristics of the electron dynamics are explored under different values of the driving electric field, lattice temperature and impurity density. Simulation results are presented by following a question-driven path of exploration, starting from the validation of the model and moving up to reasoned inquiries about the observed characteristics of electron dynamics. Our inquiry-driven learning path, based on numerical simulations, represents a viable example of how to integrate a traditional lecture-based teaching approach with effective learning strategies, providing science or engineering undergraduates with practical opportunities to enhance their comprehension of the physics governing the electron dynamics in semiconductors. Finally, we present a general discussion about the advantages and disadvantages of using an inquiry-based teaching approach within a learning environment based on semiconductor simulations.

Universality of local dissipation scales in buoyancy-driven turbulence.

PubMed

Zhou, Quan; Xia, Ke-Qing

2010-03-26

We report an experimental investigation of the local dissipation scale field eta in turbulent thermal convection. Our results reveal two types of universality of eta. The first one is that, for the same flow, the probability density functions (PDFs) of eta are insensitive to turbulent intensity and large-scale inhomogeneity and anisotropy of the system. The second is that the small-scale dissipation dynamics in buoyancy-driven turbulence can be described by the same models developed for homogeneous and isotropic turbulence. However, the exact functional form of the PDF of the local dissipation scale is not universal with respect to different types of flows, but depends on the integral-scale velocity boundary condition, which is found to have an exponential, rather than Gaussian, distribution in turbulent Rayleigh-Bénard convection.

Nonharmonicity in vibrated granular solids

NASA Astrophysics Data System (ADS)

Schreck, Carl

2012-02-01

We have shown that granular packings composed of frictionless particles with repulsive contact interactions are strongly nonharmonic. When infinitesimally perturbed along linear response eigenmodes of the static packing, energy leaks from the original mode of vibration to a continuum of frequencies due solely to contact breaking even when the system is under significant compression. Further, vibrated packings possess well-defined equilibrium positions that are different than those of the unperturbed packing. The vibrational density of states obtained using the displacement matrix and velocity autocorrelation function methods exhibit an increase in the number of low-frequency modes over that obtained from linear response of the static packing. The form of the density of states in vibrated granular packings is reminiscent of the low-frequency behavior of the vibrational density of states in fluid systems. We also investigate the effects of inter-particle friction, dissipation, particle shape, and degree of positional order on the density of states and thermal transport properties in driven granular packings.

Pattern formation of Rayleigh-Bénard convection of cold water near its density maximum in a vertical cylindrical container.

PubMed

Li, You-Rong; Ouyang, Yu-Qing; Hu, Yu-Peng

2012-10-01

In order to understand the onset of convective instability and multiple stable convection patterns of buoyancy-driven convection of cold water near its density maximum in a vertical cylindrical container heated from below, a series of three-dimensional numerical simulations were performed. The aspect ratio of the container was 2 and Prandtl number of cold water was 11.57. The sidewall was considered to be perfectly adiabatic, and the density inversion parameter was fixed at 0.3. The result shows that the density inversion phenomenon in cold water has an important effect on the critical Rayleigh number for the onset of convection and the pattern formation at higher Rayleigh numbers. When the Rayleigh number varies from 3×10(3) to 1.2×10(5), eight stable, steady convection patterns are obtained under different initial conditions. The coexistence of multiple stable steady flow patterns is also observed within some specific ranges of the Rayleigh number.

New Model for Ionospheric Irregularities at Mars

NASA Astrophysics Data System (ADS)

Keskinen, M. J.

2018-03-01

A new model for ionospheric irregularities at Mars is presented. It is shown that wind-driven currents in the dynamo region of the Martian ionosphere can be unstable to the electromagnetic gradient drift instability. This plasma instability can generate ionospheric density and magnetic field irregularities with scale sizes of approximately 15-20 km down to a few kilometers. We show that the instability-driven magnetic field fluctuation amplitudes relative to background are correlated with the ionospheric density fluctuation amplitudes relative to background. Our results can explain recent observations made by the Mars Atmosphere and Volatile EvolutioN spacecraft in the Martian ionosphere dynamo region.

Dynamics of A + B --> C reaction fronts in the presence of buoyancy-driven convection.

PubMed

Rongy, L; Trevelyan, P M J; De Wit, A

2008-08-22

The dynamics of A+B-->C fronts in horizontal solution layers can be influenced by buoyancy-driven convection as soon as the densities of A, B, and C are not all identical. Such convective motions can lead to front propagation even in the case of equal diffusion coefficients and initial concentration of reactants for which reaction-diffusion (RD) scalings predict a nonmoving front. We show theoretically that the dynamics in the presence of convection can in that case be predicted solely on the basis of the knowledge of the one-dimensional RD density profile across the front.

Laser-driven plasma photonic crystals for high-power lasers

NASA Astrophysics Data System (ADS)

Lehmann, G.; Spatschek, K. H.

2017-05-01

Laser-driven plasma density gratings in underdense plasma are shown to act as photonic crystals for high power lasers. The gratings are created by counterpropagating laser beams that trap electrons, followed by ballistic ion motion. This leads to strong periodic plasma density modulations with a lifetime on the order of picoseconds. The grating structure is interpreted as a plasma photonic crystal time-dependent property, e.g., the photonic band gap width. In Maxwell-Vlasov and particle-in-cell simulations it is demonstrated that the photonic crystals may act as a frequency filter and mirror for ultra-short high-power laser pulses.

Jet outflow and open field line measurements on the C-2U advanced beam-driven field-reversed configuration plasma experiment.

PubMed

Sheftman, D; Gupta, D; Roche, T; Thompson, M C; Giammanco, F; Conti, F; Marsili, P; Moreno, C D

2016-11-01

Knowledge and control of the axial outflow of plasma particles and energy along open-magnetic-field lines are of crucial importance to the stability and longevity of the advanced beam-driven field-reversed configuration plasma. An overview of the diagnostic methods used to perform measurements on the open field line plasma on C-2U is presented, including passive Doppler impurity spectroscopy, microwave interferometry, and triple Langmuir probe measurements. Results of these measurements provide the jet ion temperature and axial velocity, electron density, and high frequency density fluctuations.

Jet outflow and open field line measurements on the C-2U advanced beam-driven field-reversed configuration plasma experiment

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sheftman, D., E-mail: dsheftman@trialphaenergy.com; Gupta, D.; Roche, T.

Knowledge and control of the axial outflow of plasma particles and energy along open-magnetic-field lines are of crucial importance to the stability and longevity of the advanced beam-driven field-reversed configuration plasma. An overview of the diagnostic methods used to perform measurements on the open field line plasma on C-2U is presented, including passive Doppler impurity spectroscopy, microwave interferometry, and triple Langmuir probe measurements. Results of these measurements provide the jet ion temperature and axial velocity, electron density, and high frequency density fluctuations.

Taphonomic and paleoecological change in the large mammal sequence from Boomplaas Cave, western Cape, South Africa.

PubMed

Faith, J Tyler

2013-12-01

Excavations conducted by H.J. Deacon in the 1970s at Boomplaas Cave (BPA) uncovered a stratified sequence of Middle Stone Age (MSA) and Later Stone Age (LSA) deposits spanning the last >65,000 years. This study provides the first comprehensive and integrated taphonomic and paleoecological analysis of the BPA large mammals, with a focus on its implications for understanding human adaptations and environmental changes in southern Africa's Cape Floristic Region (CFR), an area that features prominently in understanding modern human origins. Taphonomic data indicate a complex history of human, carnivore, and raptor accumulation of the large mammal assemblage. The anthropogenic signal is largely absent from the bottom of the sequence (>65,000 years ago), intermediate in MSA and LSA assemblages from ~50,000 to 20,000 years ago, and strong in LSA deposits post-dating the Last Glacial Maximum (LGM). When viewed in the broader CFR context, the inferred occupation history of BPA is consistent with the hypothesis that both MSA and LSA human populations were concentrated on the submerged coastline from ~60,000 to ~20,000 years ago. Intensive occupation following the LGM parallels an apparent increase in regional population densities, which may have been driven in part by rising sea levels. The BPA ungulate assemblage is characterized by the rise and decline of a taxonomically diverse grazing community, which peaks during the LGM. These changes are not correlated with taphonomic shifts, meaning that they are likely driven by environmental factors, namely the expansion and contraction of grassland habitats. Changes in ungulate diversity indicate that effective precipitation was highest during the LGM, corresponding with an intensified winter rainfall system. This is consistent with recent arguments that the LGM in this region may not have been extremely harsh and arid. Copyright © 2013 Elsevier Ltd. All rights reserved.

Dynamic characterization of external and internal mass transport in heterotrophic biofilms from microsensors measurements.

PubMed

Guimerà, Xavier; Dorado, Antonio David; Bonsfills, Anna; Gabriel, Gemma; Gabriel, David; Gamisans, Xavier

2016-10-01

Knowledge of mass transport mechanisms in biofilm-based technologies such as biofilters is essential to improve bioreactors performance by preventing mass transport limitation. External and internal mass transport in biofilms was characterized in heterotrophic biofilms grown on a flat plate bioreactor. Mass transport resistance through the liquid-biofilm interphase and diffusion within biofilms were quantified by in situ measurements using microsensors with a high spatial resolution (<50 μm). Experimental conditions were selected using a mathematical procedure based on the Fisher Information Matrix to increase the reliability of experimental data and minimize confidence intervals of estimated mass transport coefficients. The sensitivity of external and internal mass transport resistances to flow conditions within the range of typical fluid velocities over biofilms (Reynolds numbers between 0.5 and 7) was assessed. Estimated external mass transfer coefficients at different liquid phase flow velocities showed discrepancies with studies considering laminar conditions in the diffusive boundary layer near the liquid-biofilm interphase. The correlation of effective diffusivity with flow velocities showed that the heterogeneous structure of biofilms defines the transport mechanisms inside biofilms. Internal mass transport was driven by diffusion through cell clusters and aggregates at Re below 2.8. Conversely, mass transport was driven by advection within pores, voids and water channels at Re above 5.6. Between both flow velocities, mass transport occurred by a combination of advection and diffusion. Effective diffusivities estimated at different biofilm densities showed a linear increase of mass transport resistance due to a porosity decrease up to biofilm densities of 50 g VSS·L(-1). Mass transport was strongly limited at higher biofilm densities. Internal mass transport results were used to propose an empirical correlation to assess the effective diffusivity within biofilms considering the influence of hydrodynamics and biofilm density. Copyright © 2016 Elsevier Ltd. All rights reserved.

Implementation of density-based solver for all speeds in the framework of OpenFOAM

NASA Astrophysics Data System (ADS)

Shen, Chun; Sun, Fengxian; Xia, Xinlin

2014-10-01

In the framework of open source CFD code OpenFOAM, a density-based solver for all speeds flow field is developed. In this solver the preconditioned all speeds AUSM+(P) scheme is adopted and the dual time scheme is implemented to complete the unsteady process. Parallel computation could be implemented to accelerate the solving process. Different interface reconstruction algorithms are implemented, and their accuracy with respect to convection is compared. Three benchmark tests of lid-driven cavity flow, flow crossing over a bump, and flow over a forward-facing step are presented to show the accuracy of the AUSM+(P) solver for low-speed incompressible flow, transonic flow, and supersonic/hypersonic flow. Firstly, for the lid driven cavity flow, the computational results obtained by different interface reconstruction algorithms are compared. It is indicated that the one dimensional reconstruction scheme adopted in this solver possesses high accuracy and the solver developed in this paper can effectively catch the features of low incompressible flow. Then via the test cases regarding the flow crossing over bump and over forward step, the ability to capture characteristics of the transonic and supersonic/hypersonic flows are confirmed. The forward-facing step proves to be the most challenging for the preconditioned solvers with and without the dual time scheme. Nonetheless, the solvers described in this paper reproduce the main features of this flow, including the evolution of the initial transient.

Psychosocial Impact of Personalized Therapies in Oncology.

PubMed

Schilling, Georgia; Schulz-Kindermann, Frank

2018-01-01

Personalized medicine is a keyword in modern oncology summarizing biomarker-driven targeted therapies. Those novel agents enhance our therapeutic portfolio and offer new options for our patients. But the term is often misleading and implicates a tailored therapy to the individual person, but it rather means a treatment stratified on genetic characteristics of the tumor. Molecular therapies raise expectations of curability or long-term treatments making former life-threatening diseases to more chronic ones but this is true only for some patients. So we have to carefully communicate with our patients about the options and limitations of those modern therapies not to trigger disappointments.

Topographically induced internal solitary waves in a pycnocline: Ultrasonic probes and stereo-correlation measurements

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dossmann, Yvan, E-mail: yvan.dossmann@anu.edu.au; CNRM-GAME, UMR3589 METEO-FRANCE and CNRS, 42 avenue Gaspard Coriolis, 31057 Toulouse Cedex 01; Laboratoire d’Aérologie, 14 avenue Edouard Belin, 31400 Toulouse

Internal solitary waves (ISWs) are large amplitude stable waves propagating in regions of high density gradients such as the ocean pycnocline. Their dynamics has often been investigated in two-dimensional approaches, however, their three-dimensional evolution is still poorly known. Experiments have been conducted in the large stratified water tank of CNRM-GAME to study the generation of ISWs in two academic configurations inspired by oceanic regimes. First, ultrasonic probes are used to measure the interfacial displacement in the two configurations. In the primary generation case for which the two layers are of constant density, the generation of ISWs is investigated in twomore » series of experiments with varying amplitude and forcing frequency. In the secondary generation case for which the lower layer is stratified, the generation of ISWs from the impact of an internal wave beam on the pycnocline and their subsequent dynamics is studied. The dynamics of ISWs in these two regimes accords well with analytical approaches and numerical simulations performed in analogous configurations. Then, recent developments of a stereo correlation technique are used to describe the three-dimensional structure of propagating ISWs. In the primary generation configuration, small transverse effects are observed in the course of the ISW propagation. In the secondary generation configuration, larger transverse structures are observed in the interfacial waves dynamics. The interaction between interfacial troughs and internal waves propagating in the lower stratified layer are a possible cause for the generation of these structures. The magnitude of these transverse structures is quantified with a nondimensional parameter in the two configurations. They are twice as large in the secondary generation case as in the primary generation case.« less

Are Strong Zonal Winds in Giant Planets Caused by Density-Stratification?

NASA Astrophysics Data System (ADS)

Verhoeven, J.; Stellmach, S.

2012-12-01

One of the most striking features of giant planets like Jupiter and Saturn are the zonal wind patterns observed on their surfaces. The mechanism that drives this differential rotation is still not clearly identified and is currently strongly debated in the astro- and geophysics community. Different mechanisms have been proposed over the last decades. Here, a recently discovered mechanism based on background density stratification (Glatzmaier et al., 2009) is investigated. This mechanism has the potential to overcome known difficulties of previous explanations and its efficiency has been demonstrated in 2-d simulations covering equatorial planes. By performing highly resolved numerical simulations in a local Cartesian geometry, we are able to test the efficiency and functionality of this mechanism in turbulent, rotating convection in three spatial dimensions. The choice of a Cartesian model geometry naturally excludes other known mechanisms capable of producing differential rotation, thus allowing us to investigate the role of density stratification in isolation. Typically, the dynamics can be classified into two main regimes: A regime exhibiting strong zonal winds for weak to moderate thermal driving and a regime where zonal winds are largely absent in the case of a strong thermal forcing. Our results indicate that previous 2-d results must be handled with care and can only explain parts of the full 3-d behavior. We show that the density-stratification mechanism tends to operate in a more narrow parameter range in 3-d as compared to 2-d simulations. The dynamics of the regime transition is shown to differ in both cases, which renders scaling laws derived from two-dimensional studies questionable. Based on our results, we provide estimates for the importance of the density-stratification mechanism for giant planets like Jupiter (strong density stratification), for systems like the Earth's core (weak density stratification) and compare its efficiency with other suggested mechanisms for driving differential rotation. Gary A. Glatzmaier, Martha Evonuk and Tamara M. Rogers (2009), Differential rotation in giant planets maintained by density-stratified turbulent convection. Geophysical and Astrophysical Fluid Dynamics, Vol. 103, No. 1, 31-51.

The Intrinsic Characteristics of Galaxies on the SFR–M ∗ Plane at 1.2 < z < 4: I. The Correlation between Stellar Age, Central Density, and Position Relative to the Main Sequence

NASA Astrophysics Data System (ADS)

Lee, Bomee; Giavalisco, Mauro; Whitaker, Katherine; Williams, Christina C.; Ferguson, Henry C.; Acquaviva, Viviana; Koekemoer, Anton M.; Straughn, Amber N.; Guo, Yicheng; Kartaltepe, Jeyhan S.; Lotz, Jennifer; Pacifici, Camilla; Croton, Darren J.; Somerville, Rachel S.; Lu, Yu

2018-02-01

We use the deep CANDELS observations in the GOODS North and South fields to revisit the correlations between stellar mass (M *), star formation rate (SFR) and morphology, and to introduce a fourth dimension, the mass-weighted stellar age, in galaxies at 1.2< z< 4. We do this by making new measures of M *, SFR, and stellar age thanks to an improved SED fitting procedure that allows various star formation history for each galaxy. Like others, we find that the slope of the main sequence (MS) of star formation in the ({M}* ;{SFR}) plane bends at high mass. We observe clear morphological differences among galaxies across the MS, which also correlate with stellar age. At all redshifts, galaxies that are quenching or quenched, and thus old, have high {{{Σ }}}1 (the projected density within the central 1 kpc), while younger, star-forming galaxies span a much broader range of {{{Σ }}}1, which includes the high values observed for quenched galaxies, but also extends to much lower values. As galaxies age and quench, the stellar age and the dispersion of {{{Σ }}}1 for fixed values of M * shows two different regimes: one at the low-mass end, where quenching might be driven by causes external to the galaxies; the other at the high-mass end, where quenching is driven by internal causes, very likely the mass given the low scatter of {{{Σ }}}1 (mass quenching). We suggest that the monotonic increase of central density as galaxies grow is one manifestation of a more general phenomenon of structural transformation that galaxies undergo as they evolve.

Effects of Phytoplankton Growth Phase on Delayed Settling Behavior of Marine Snow Aggregates at Sharp Density Transitions

NASA Astrophysics Data System (ADS)

Proctor, K. W.; Montgomery, Q. W.; Prairie, J. C.

2016-02-01

Marine snow aggregates play a fundamental role in the marine carbon cycle. Since marine snow aggregates are larger and thus sink faster than individual phytoplankton, aggregates often dominate carbon flux. Previous studies have shown that marine snow aggregates will significantly decrease their settling velocity when passing through sharp density transitions within the ocean, a phenomenon defined as delayed settling. Given the importance of aggregate settling to carbon export, these small-scale changes in aggregate settling dynamics may have significant impacts on the efficiency of the biological pump. However, there is still a lack of knowledge about how different physical properties of aggregates can affect this delayed settling. In this study, we investigated the effect of phytoplankton growth phase on delayed settling behavior. Using phytoplankton cultures stopped at four different growth phases, we formed marine snow aggregates in the laboratory in rotating cylindrical tanks. We then observed individual aggregates as they settled through a stratified tank. We will present data which illustrates that aggregates experience greatly reduced settling rates when passing through sharp density gradients and that the growth phase of the phytoplankton used to form these aggregates has a significant effect on this delayed settling behavior. A thorough understanding of the impact of phytoplankton growth phase on the delayed settling behavior of marine snow will offer insight into the way phytoplankton growth phase may influence the efficiency of the biological pump, carbon flux, and the carbon cycle as a whole.

Options for refractive index and viscosity matching to study variable density flows

NASA Astrophysics Data System (ADS)

Clément, Simon A.; Guillemain, Anaïs; McCleney, Amy B.; Bardet, Philippe M.

2018-02-01

Variable density flows are often studied by mixing two miscible aqueous solutions of different densities. To perform optical diagnostics in such environments, the refractive index of the fluids must be matched, which can be achieved by carefully choosing the two solutes and the concentration of the solutions. To separate the effects of buoyancy forces and viscosity variations, it is desirable to match the viscosity of the two solutions in addition to their refractive index. In this manuscript, several pairs of index matched fluids are compared in terms of viscosity matching, monetary cost, and practical use. Two fluid pairs are studied in detail, with two aqueous solutions (binary solutions of water and a salt or alcohol) mixed into a ternary solution. In each case: an aqueous solution of isopropanol mixed with an aqueous solution of sodium chloride (NaCl) and an aqueous solution of glycerol mixed with an aqueous solution of sodium sulfate (Na_2SO_4). The first fluid pair allows reaching high-density differences at low cost, but brings a large difference in dynamic viscosity. The second allows matching dynamic viscosity and refractive index simultaneously, at reasonable cost. For each of these four solutes, the density, kinematic viscosity, and refractive index are measured versus concentration and temperature, as well as wavelength for the refractive index. To investigate non-linear effects when two index-matched, binary solutions are mixed, the ternary solutions formed are also analyzed. Results show that density and refractive index follow a linear variation with concentration. However, the viscosity of the isopropanol and NaCl pair deviates from the linear law and has to be considered. Empirical correlations and their coefficients are given to create index-matched fluids at a chosen temperature and wavelength. Finally, the effectiveness of the refractive index matching is illustrated with particle image velocimetry measurements performed for a buoyant jet in a linearly stratified environment. The creation of the index-matched solutions and linear stratification in a large-scale experimental facility are detailed, as well as the practical challenges to obtain precise refractive index matching.

Efficient cold outflows driven by cosmic rays in high-redshift galaxies and their global effects on the IGM

NASA Astrophysics Data System (ADS)

Samui, Saumyadip; Subramanian, Kandaswamy; Srianand, Raghunathan

2018-05-01

We present semi-analytical models of galactic outflows in high-redshift galaxies driven by both hot thermal gas and non-thermal cosmic rays. Thermal pressure alone may not sustain a large-scale outflow in low-mass galaxies (i.e. M ˜ 108 M⊙), in the presence of supernovae feedback with large mass loading. We show that inclusion of cosmic ray pressure allows outflow solutions even in these galaxies. In massive galaxies for the same energy efficiency, cosmic ray-driven winds can propagate to larger distances compared to pure thermally driven winds. On an average gas in the cosmic ray-driven winds has a lower temperature which could aid detecting it through absorption lines in the spectra of background sources. Using our constrained semi-analytical models of galaxy formation (that explains the observed ultraviolet luminosity functions of galaxies), we study the influence of cosmic ray-driven winds on the properties of the intergalactic medium (IGM) at different redshifts. In particular, we study the volume filling factor, average metallicity, cosmic ray and magnetic field energy densities for models invoking atomic cooled and molecular cooled haloes. We show that the cosmic rays in the IGM could have enough energy that can be transferred to the thermal gas in presence of magnetic fields to influence the thermal history of the IGM. The significant volume filling and resulting strength of IGM magnetic fields can also account for recent γ-ray observations of blazars.

Evolution in population parameters: density-dependent selection or density-dependent fitness?

PubMed

Travis, Joseph; Leips, Jeff; Rodd, F Helen

2013-05-01

Density-dependent selection is one of earliest topics of joint interest to both ecologists and evolutionary biologists and thus occupies an important position in the histories of these disciplines. This joint interest is driven by the fact that density-dependent selection is the simplest form of feedback between an ecological effect of an organism's own making (crowding due to sustained population growth) and the selective response to the resulting conditions. This makes density-dependent selection perhaps the simplest process through which we see the full reciprocity between ecology and evolution. In this article, we begin by tracing the history of studying the reciprocity between ecology and evolution, which we see as combining the questions of evolutionary ecology with the assumptions and approaches of ecological genetics. In particular, density-dependent fitness and density-dependent selection were critical concepts underlying ideas about adaptation to biotic selection pressures and the coadaptation of interacting species. However, theory points to a critical distinction between density-dependent fitness and density-dependent selection in their influences on complex evolutionary and ecological interactions among coexisting species. Although density-dependent fitness is manifestly evident in empirical studies, evidence of density-dependent selection is much less common. This leads to the larger question of how prevalent and important density-dependent selection might really be. Life-history variation in the least killifish Heterandria formosa appears to reflect the action of density-dependent selection, and yet compelling evidence is elusive, even in this well-studied system, which suggests some important challenges for understanding density-driven feedbacks between ecology and evolution.

Improving the accuracy of livestock distribution estimates through spatial interpolation.

PubMed

Bryssinckx, Ward; Ducheyne, Els; Muhwezi, Bernard; Godfrey, Sunday; Mintiens, Koen; Leirs, Herwig; Hendrickx, Guy

2012-11-01

Animal distribution maps serve many purposes such as estimating transmission risk of zoonotic pathogens to both animals and humans. The reliability and usability of such maps is highly dependent on the quality of the input data. However, decisions on how to perform livestock surveys are often based on previous work without considering possible consequences. A better understanding of the impact of using different sample designs and processing steps on the accuracy of livestock distribution estimates was acquired through iterative experiments using detailed survey. The importance of sample size, sample design and aggregation is demonstrated and spatial interpolation is presented as a potential way to improve cattle number estimates. As expected, results show that an increasing sample size increased the precision of cattle number estimates but these improvements were mainly seen when the initial sample size was relatively low (e.g. a median relative error decrease of 0.04% per sampled parish for sample sizes below 500 parishes). For higher sample sizes, the added value of further increasing the number of samples declined rapidly (e.g. a median relative error decrease of 0.01% per sampled parish for sample sizes above 500 parishes. When a two-stage stratified sample design was applied to yield more evenly distributed samples, accuracy levels were higher for low sample densities and stabilised at lower sample sizes compared to one-stage stratified sampling. Aggregating the resulting cattle number estimates yielded significantly more accurate results because of averaging under- and over-estimates (e.g. when aggregating cattle number estimates from subcounty to district level, P <0.009 based on a sample of 2,077 parishes using one-stage stratified samples). During aggregation, area-weighted mean values were assigned to higher administrative unit levels. However, when this step is preceded by a spatial interpolation to fill in missing values in non-sampled areas, accuracy is improved remarkably. This counts especially for low sample sizes and spatially even distributed samples (e.g. P <0.001 for a sample of 170 parishes using one-stage stratified sampling and aggregation on district level). Whether the same observations apply on a lower spatial scale should be further investigated.

A New Stratified Sampling Procedure which Decreases Error Estimation of Varroa Mite Number on Sticky Boards.

PubMed

Kretzschmar, A; Durand, E; Maisonnasse, A; Vallon, J; Le Conte, Y

2015-06-01

A new procedure of stratified sampling is proposed in order to establish an accurate estimation of Varroa destructor populations on sticky bottom boards of the hive. It is based on the spatial sampling theory that recommends using regular grid stratification in the case of spatially structured process. The distribution of varroa mites on sticky board being observed as spatially structured, we designed a sampling scheme based on a regular grid with circles centered on each grid element. This new procedure is then compared with a former method using partially random sampling. Relative error improvements are exposed on the basis of a large sample of simulated sticky boards (n=20,000) which provides a complete range of spatial structures, from a random structure to a highly frame driven structure. The improvement of varroa mite number estimation is then measured by the percentage of counts with an error greater than a given level. © The Authors 2015. Published by Oxford University Press on behalf of Entomological Society of America. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

Epidemiological Implications of Host Biodiversity and Vector Biology: Key Insights from Simple Models.

PubMed

Dobson, Andrew D M; Auld, Stuart K J R

2016-04-01

Models used to investigate the relationship between biodiversity change and vector-borne disease risk often do not explicitly include the vector; they instead rely on a frequency-dependent transmission function to represent vector dynamics. However, differences between classes of vector (e.g., ticks and insects) can cause discrepancies in epidemiological responses to environmental change. Using a pair of disease models (mosquito- and tick-borne), we simulated substitutive and additive biodiversity change (where noncompetent hosts replaced or were added to competent hosts, respectively), while considering different relationships between vector and host densities. We found important differences between classes of vector, including an increased likelihood of amplified disease risk under additive biodiversity change in mosquito models, driven by higher vector biting rates. We also draw attention to more general phenomena, such as a negative relationship between initial infection prevalence in vectors and likelihood of dilution, and the potential for a rise in density of infected vectors to occur simultaneously with a decline in proportion of infected hosts. This has important implications; the density of infected vectors is the most valid metric for primarily zoonotic infections, while the proportion of infected hosts is more relevant for infections where humans are a primary host.

Rabi-Bloch oscillations in spatially distributed systems: Temporal dynamics and frequency spectra

NASA Astrophysics Data System (ADS)

Levie, Ilay; Kastner, Raphael; Slepyan, Gregory

2017-10-01

We consider one-dimensional chains of two-level quantum systems coupled via tunneling. The chain is driven by the superposition of dc and ac fields in the strong coupling regime. Based on the fundamental principles of electrodynamics and quantum theory, we have developed a generalized model of quantum dynamics for such interactions, free of rotating-wave approximation. The system of equations of motion was studied numerically. We analyzed the dynamics and spectra of the inversion density, dipole current density, and tunneling current density. In the case of resonant interaction with the ac component, the particle dynamics exhibits itself in the oscillatory regime, which may be interpreted as a combination of Rabi and Bloch oscillations with their strong mutual influence. Such scenario for an obliquely incident ac field dramatically differs from the individual picture of both types of oscillations due to the interactions. This effect is counterintuitive because of the existence of markedly different frequency ranges for such two types of oscillations. These dynamics manifest themselves in multiline spectra in different combinations of Rabi and Bloch frequencies. The effect is promising as a framework of a new type of spectroscopy in nanoelectronics and electrical control of nanodevices.

Mixing and diffusion in intermittent overturning turbulence

NASA Astrophysics Data System (ADS)

Redondo, Jose M.; Mahjoub, Otman B.; Gonzalez-Nieto, Pilar L.; Lawry, Andrew

2014-05-01

The improvements in experimental methods and high resolution image analysis are nowadays able to detect subtle changes in the structure of the turbulence over a wide range of temporal and spatial scales [1], we compare the scaling shown by different mixing fronts driven by buoyancy that form a Rayleigh-Taylor mixing front. We use PIV and density front tracking in several experimental configurations akin to geophysical overturning [2-7]. We parametrize the role of unstable stratification by means of the Atwood number and compare both the scaling and the multifractal and the maximum local fractal structure functions of the different markers used to visualize the front. Both reactive and passive scalar tracers are used to investigate the mixing structure and the intermittency of the flow. Different initial conditions are compared and the mixing efficiency of the overal turbulent process evaluated [6-7]. An interesting approach, relating the Multi-Fractal dimension spectra, the intermittency and the spectral exponent is to find relationships that may be used to parameterise the sub-grid turbulence in terms of generalized diffusivities [4 ] that take into account the topology and the self-similarity of the Mixing RT and RM flows. As an example, a relationship between the diffusivity, the exponent β, the intermittency μ, and D(i), may be found for the volume fraction or the concentration, at the same time other locally measured parameters such as the enstrophy or the gradient alignment as well as their multi-fractal structures may turn out to be physically relevant indicators of the local turbulence and the mixing. Several methods of deriving local eddy diffusivity and local entrainment should give more realistic estimates of the spatial/temporal non-homogeneities (and intermittencies in the Kolmogorov 62 sense obtained as spatial correlations of the turbulent dissipation, or from structure functions) and these values may be used to parameterise turbulence at a variety of scales. The method involving the multi-fractal dimension measurements is much more elaborated and seems to have a better theoretical justification in the sense that it is possible that different concentrations showing different fractal dimensions may be due to different levels of intermittency (and thus different spectra, which are not generally in equilibrium as dscribed by[9,10]. Using topological descriptors we can establish now a theoretical baseline pattern for the turbulence behaviour that is reflected in the different structures (volume fraction, velocity, vorticity, helicity) we can thus obtain a classification relating D3 and the integral of the different fractal dimensions D2 for different levels of scalar (volume fraction reaction intensity or temperature). [5,8,11] Vorticity evolution is more smooth and quite different than that of volume fraction or density and these seem also different for the RT and RM instability driven mixing showing a wider range of even higher mixing efficiencies 0- 0.66 Thanks to European Union project ERBIC15-CT96-0111 Multi-scale complex fluid flows and interfacial phenomena (PITN-GA-2008-214919). ERCOFTAC and GenCat. grant 2001SGR00221. [1] Mahjoub O.B., Redondo J.M. and Babiano A. (2000) Hyerarchy flux in nonhomogeneous flows in Turbulent diffusion in the environment Eds. Redondo J.M. and Babiano A. 249-260. . [2] Dalziel, S. B.,(1994) Perturbations and coherent flow in Rayleigh-Taylor instability: in 4th International Workshop on the Physics of Compressible Turbulent Mixing, ed. P. F. Linden, D. L. Youngs, & S. B. Dalziel; 32-41. [3] Linden, P. F., Redondo, J. M., and Youngs, D. (1994) Molecular mixing in Rayleigh-Taylor instability, J. Fluid Mech. 265, 97-124 [4] Vindel, J.M., Yague, C. and Redondo, J.M. Nuovo Cimento (2008) 31, [5]Redondo J.M. (1993) Fractal models of density interfaces. Wavelets, Fractals and Fourier transforms. (Eds.) M. Farge, J.C.R. Hunt and J.C. Vassilicos. 353-370. IMA number 43, Clarendon Press, Oxford 1993 [6] J.M Redondo, J.M. Grau, A. Platonov, G. Garzon, (2008) Rev. Int. Met. Num. Ing. 24, 23. [7] Fraunie P., Berreba S. Chashechkin Y., Velasco D. and Redondo J.M. (2008) LES and laboratory experiments on the decay of grid wakes in strongly stratified flows. Il Nuovo Cimento C 31, 909-930. [8] Redondo J.M. (2002) "Mixing efficiencies of different kinds of turbulent processes and instabilities, Applications to the environment in Turbulent mixing in geophysical flows". Eds. Linden P.F. and Redondo J.M. 131-157. [9] Redondo J.M. (1990). The structure of density interfaces, Ph. D. Thesis, DAMTP University of Cambridge. [10]Redondo J.M. (1987). Effects of ground proximity on dense gas entrainment, Journal of Hazardous Materials, 16, 381-393. [11] Lopez, P., Cano, J.L. and Redondo, J.M.(2008) An experimental model of mixing processes generated by an array of top-heavy turbulent plumes, Il Nuovo Cimento, 31C, 679-698.

Combining density functional theory calculations, supercomputing, and data-driven methods to design new materials (Conference Presentation)

NASA Astrophysics Data System (ADS)

Jain, Anubhav

2017-04-01

Density functional theory (DFT) simulations solve for the electronic structure of materials starting from the Schrödinger equation. Many case studies have now demonstrated that researchers can often use DFT to design new compounds in the computer (e.g., for batteries, catalysts, and hydrogen storage) before synthesis and characterization in the lab. In this talk, I will focus on how DFT calculations can be executed on large supercomputing resources in order to generate very large data sets on new materials for functional applications. First, I will briefly describe the Materials Project, an effort at LBNL that has virtually characterized over 60,000 materials using DFT and has shared the results with over 17,000 registered users. Next, I will talk about how such data can help discover new materials, describing how preliminary computational screening led to the identification and confirmation of a new family of bulk AMX2 thermoelectric compounds with measured zT reaching 0.8. I will outline future plans for how such data-driven methods can be used to better understand the factors that control thermoelectric behavior, e.g., for the rational design of electronic band structures, in ways that are different from conventional approaches.

Tunneling anisotropic magnetoresistance driven by magnetic phase transition.

PubMed

Chen, X Z; Feng, J F; Wang, Z C; Zhang, J; Zhong, X Y; Song, C; Jin, L; Zhang, B; Li, F; Jiang, M; Tan, Y Z; Zhou, X J; Shi, G Y; Zhou, X F; Han, X D; Mao, S C; Chen, Y H; Han, X F; Pan, F

2017-09-06

The independent control of two magnetic electrodes and spin-coherent transport in magnetic tunnel junctions are strictly required for tunneling magnetoresistance, while junctions with only one ferromagnetic electrode exhibit tunneling anisotropic magnetoresistance dependent on the anisotropic density of states with no room temperature performance so far. Here, we report an alternative approach to obtaining tunneling anisotropic magnetoresistance in α'-FeRh-based junctions driven by the magnetic phase transition of α'-FeRh and resultantly large variation of the density of states in the vicinity of MgO tunneling barrier, referred to as phase transition tunneling anisotropic magnetoresistance. The junctions with only one α'-FeRh magnetic electrode show a magnetoresistance ratio up to 20% at room temperature. Both the polarity and magnitude of the phase transition tunneling anisotropic magnetoresistance can be modulated by interfacial engineering at the α'-FeRh/MgO interface. Besides the fundamental significance, our finding might add a different dimension to magnetic random access memory and antiferromagnet spintronics.Tunneling anisotropic magnetoresistance is promising for next generation memory devices but limited by the low efficiency and functioning temperature. Here the authors achieved 20% tunneling anisotropic magnetoresistance at room temperature in magnetic tunnel junctions with one α'-FeRh magnetic electrode.

Evaluation of lightning accommodation systems for wind-driven turbine rotors

NASA Technical Reports Server (NTRS)

Bankaitis, H.

1982-01-01

Wind-driven turbine generators are being evaluated as an alternative source of electric energy. Areas of favorable location for the wind-driven turbines (high wind density) coincide with areas of high incidence of thunderstorm activity. These locations, coupled with the 30-m or larger diameter rotor blades, make the wind-driven turbine blades probable terminations for lightning strikes. Several candidate systems of lightning accommodation for composite-structural-material blades were designed and their effectiveness evaluated by submitting the systems to simulated lightning strikes. The test data were analyzed and system design were reviewed on the basis of the analysis.

Shock interaction with a two-gas interface in a novel dual-driver shock tube

NASA Astrophysics Data System (ADS)

Labenski, John R.

Fluid instabilities exist at the interface between two fluids having different densities if the flow velocity and density gradient are anti-parallel or if a shock wave crosses the boundary. The former case is called the Rayleigh-Taylor (R-T) instability and the latter, the Richtmyer-Meshkov (R-M) instability. Small initial perturbations on the interface destabilize and grow into larger amplitude structures leading to turbulent mixing. Instabilities of this type are seen in inertial confinement fusion (ICF) experiments, laser produced plasmas, supernova explosions, and detonations. A novel dual-driver shock tube was used to investigate the growth rate of the R-M instability. One driver is used to create an argon-refrigerant interface, and the other at the opposite end of the driven section generates a shock to force the interface with compressible flows behind the shock. The refrigerant gas in the first driver is seeded with sub-micron oil droplets for visualization of the interface. The interface travels down the driven section past the test section for a fixed amount of time. A stronger shock of Mach 1.1 to 1.3 drives the interface back past the test section where flow diagnostics are positioned. Two schlieren systems record the density fluctuations while light scattering detectors record the density of the refrigerant as a function of position over the interface. A pair of digital cameras take stereo images of the interface, as mapped out by the tracer particles under illumination by a Q-switched ruby laser. The amount of time that the interface is allowed to travel up the driven section determines the interaction time as a control. Comparisons made between the schlieren signals, light scattering detector outputs, and the images quantify the fingered characteristics of the interface and its growth due to shock forcing. The results show that the interface has a distribution of thickness and that the interaction with a shock further broadens the interface. The growth rate was found to exhibit a dependence on the shock strength.

Influence of driven current on resistive tearing mode in Tokamaks

NASA Astrophysics Data System (ADS)

Ma, Zhiwei; Wang, Sheng; Zhang, Wei

2016-10-01

Influence of driven current on the m / n = 2 / 1 resistive tearing mode is studied systematically using a three-dimensional toroidal MHD code (CLT). A uniform driven current with Gaussian distribution in the radial direction is imposed around the unperturbed rational surface. It is found that the driven current can locally modify the profiles of the current and safety factor, such that the tearing mode becomes linearly stable. The stabilizing effect increases with increase of the driven current Icd or decrease of its width δcd, unless an excessively large driven current reverses the magnetic shear near the rational surface and drives other instabilities such as double or triple tearing modes. The stabilizing effect can be negligible or becomes reversed if the maximum driven current density is not at the unperturbed rational surface. ITER-CN Program.

How we compute N matters to estimates of mixing in stratified flows

DOE Office of Scientific and Technical Information (OSTI.GOV)

Arthur, Robert S.; Venayagamoorthy, Subhas K.; Koseff, Jeffrey R.

We know that most commonly used models for turbulent mixing in the ocean rely on a background stratification against which turbulence must work to stir the fluid. While this background stratification is typically well defined in idealized numerical models, it is more difficult to capture in observations. Here, a potential discrepancy in ocean mixing estimates due to the chosen calculation of the background stratification is explored using direct numerical simulation data of breaking internal waves on slopes. There are two different methods for computing the buoyancy frequencymore » $N$$, one based on a three-dimensionally sorted density field (often used in numerical models) and the other based on locally sorted vertical density profiles (often used in the field), are used to quantify the effect of$$N$$on turbulence quantities. It is shown that how$$N$$is calculated changes not only the flux Richardson number$$R_{f}$$, which is often used to parameterize turbulent mixing, but also the turbulence activity number or the Gibson number$$Gi$$, leading to potential errors in estimates of the mixing efficiency using$$Gi$-based parameterizations.« less

How we compute N matters to estimates of mixing in stratified flows

DOE PAGES

Arthur, Robert S.; Venayagamoorthy, Subhas K.; Koseff, Jeffrey R.; ...

2017-10-13

We know that most commonly used models for turbulent mixing in the ocean rely on a background stratification against which turbulence must work to stir the fluid. While this background stratification is typically well defined in idealized numerical models, it is more difficult to capture in observations. Here, a potential discrepancy in ocean mixing estimates due to the chosen calculation of the background stratification is explored using direct numerical simulation data of breaking internal waves on slopes. There are two different methods for computing the buoyancy frequencymore » $N$$, one based on a three-dimensionally sorted density field (often used in numerical models) and the other based on locally sorted vertical density profiles (often used in the field), are used to quantify the effect of$$N$$on turbulence quantities. It is shown that how$$N$$is calculated changes not only the flux Richardson number$$R_{f}$$, which is often used to parameterize turbulent mixing, but also the turbulence activity number or the Gibson number$$Gi$$, leading to potential errors in estimates of the mixing efficiency using$$Gi$-based parameterizations.« less

Double hydrophilic block copolymer controlled growth and self-assembly of CaCO3 multilayered structures at the air/water interface.

PubMed

Gao, Yun-Xiang; Yu, Shu-Hong; Guo, Xiao-Hui

2006-07-04

Double hydrophilic block copolymers PEG-b-PEI-linear with different PEI block lengths have been examined for CaCO3 mineralization at the air/water interface. The results demonstrated that either PEI length or the solution acidity had a significant influence on the morphogenesis of vaterite crystals at the air/water interface. A possible mechanism for the stratification of CaCO3 vaterite crystals has been proposed. Increasing either PEI length or the initial pH value of the solution will decrease the density of the PEG block anchored on the binding interface and result in exposing more space as binding interface to solution and favoring the subnucleation and stratification growth on the polymer-CaCO3 interface. In contrast, higher density of PEG blocks will stabilize the growing crystals more efficiently and inhibit subnucleation on the polymer-CaCO3 interface, and thus prevent the formation of stratified structures. This study provides an example that it is possible to access morphogenesis of calcium carbonate structures by a combination of a block copolymer with the air/water interface.

Status and trends of native birds in the Keauhou and Kilauea forest, Hawai`i Island.

USGS Publications Warehouse

Camp, Richard J.; Jacobi, James D.; Pratt, Thane K.; Gorresen, P. Marcos; Rubenstein, Tanya

2010-01-01

A Safe Harbor Agreement (SHA) is a voluntary arrangement between the U.S. Fish and Wildlife Service and non-Federal landowners to promote the protection, conservation, and recovery of listed species without imposing further land use restrictions on the landowners. Kamehameha Schools is considering entering into a SHA for their Keauhou and Kīlauea Forest lands on the island of Hawai′i. Bird surveys were conducted in 2008 to determine the current occurrence and density of listed species for the Keauhou and Kīlauea Forest, a prerequisite for establishing an agreement. Because of different management practices in the proposed SHA area we stratified the survey data into intact and altered forest strata. The listed passerines—′Akiapōlā′au (Hemignathus munroi), Hawai′i Creeper (Oreomystis mana), and Hawai′i ′Ākepa (Loxops coccineus)—occur in both strata but at low densities. The endangered ′Io (Hawaiian Hawk; Buteo solitarius) also occurs within both strata at low densities. This report was prepared for the U.S. Fish and Wildlife Service and Kamehameha Schools to provide information they can use to establish baseline levels for the SHA. In addition, we describe the status and trends of the non-listed native birds.

Non-linear vibrational response of Ge and SiC membranes

NASA Astrophysics Data System (ADS)

Zhou, L. Q.; Colston, G.; Pearce, M. J.; Prince, R. G.; Myronov, M.; Leadley, D. R.; Trushkevych, O.; Edwards, R. S.

2017-07-01

Characterisation of membranes produced for use as micro-electro-mechanical systems using vibrational techniques can give a measure of their behaviour and suitability for operation in different environments. Two membranes are studied here: germanium (Ge) and cubic silicon carbide (3C-SiC) on a silicon (Si) substrate. When driven at higher displacements, the membranes exhibit self-protecting behaviour. The resonant vibration amplitude is limited to a maximum value of around 10 nm, through dissipation of energy via higher harmonic vibrations. This is observed for both materials, despite their different Young's moduli and defect densities.

The influence of surface properties on the plasma dynamics in radio-frequency driven oxygen plasmas: Measurements and simulations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Greb, Arthur; Niemi, Kari; O'Connell, Deborah

2013-12-09

Plasma parameters and dynamics in capacitively coupled oxygen plasmas are investigated for different surface conditions. Metastable species concentration, electronegativity, spatial distribution of particle densities as well as the ionization dynamics are significantly influenced by the surface loss probability of metastable singlet delta oxygen (SDO). Simulated surface conditions are compared to experiments in the plasma-surface interface region using phase resolved optical emission spectroscopy. It is demonstrated how in-situ measurements of excitation features can be used to determine SDO surface loss probabilities for different surface materials.

Amorphous ices explained in terms of nonequilibrium phase transitions in supercooled water

NASA Astrophysics Data System (ADS)

Limmer, David; Chandler, David

2013-03-01

We analyze the phase diagram of supercooled water out-of-equilibrium using concepts from space-time thermodynamics and the dynamic facilitation theory of the glass transition, together with molecular dynamics simulations. We find that when water is driven out-of-equilibrium, it can exist in multiple amorphous states. In contrast, we find that when water is at equilibrium, it can exist in only one liquid state. The amorphous non-equilibrium states are solids, distinguished from the liquid by their lack of mobility, and distinguished from each other by their different densities and local structure. This finding explains the experimentally observed polyamorphism of water as a class of nonequilibrium phenomena involving glasses of different densities. While the amorphous solids can be long lived, they are thermodynamically unstable. When allowed to relax to equilibrium, they crystallize with pathways that pass first through liquid state configurations and then to ordered ice.

Role of density gradient driven trapped electron mode turbulence in the H-mode inner core with electron heating

DOE PAGES

Ernst, D. R.; Burrell, K. H.; Guttenfelder, W.; ...

2016-05-10

In a series of DIII-D [J. L. Luxon, Nucl. Fusion 42 614 (2002)] low torque quiescent H-mode experiments show that density gradient driven TEM (DGTEM) turbulence dominates the inner core of H-Mode plasmas during strong electron cyclotron heating (ECH). By adding 3.4 MW ECH doubles T e/T i from 0.5 to 1.0, which halves the linear DGTEM critical density gradient, locally reducing density peaking, while transport in all channels displays extreme stiffness in the density gradient. This then suggests fusion -heating may degrade inner core confinement in H-Mode plasmas with moderate density peaking and low collisionality, with equal electron andmore » ion temperatures, key conditions expected in burning plasmas. Gyrokinetic simulations using GYRO [J. Candy and R. E. Waltz, J. Comp. Phys. 186 545 (2003)] (and GENE [F. Jenko et al., Phys. Plasmas 7, 1904 (2000)]) closely match not only particle, energy, and momentum fluxes, but also density fluctuation spectra from Doppler Backscattering (DBS), with and without ECH. Inner core DBS density fluctuations display discrete frequencies with adjacent toroidal mode numbers, which we identify as DGTEMs. GS2 [W. Dorland et al., Phys. Rev. Lett. 85 5579 (2000)] predictions show the DGTEM can be suppressed, to avoid degradation with electron heating, by broadening the current density profile to attain q 0 > q min > 1.« less

Ethnic density and depressive symptoms among African Americans: threshold and differential effects across social and demographic subgroups.

PubMed

Bécares, Laia; Nazroo, James; Jackson, James

2014-12-01

We examined the association between Black ethnic density and depressive symptoms among African Americans. We sought to ascertain whether a threshold exists in the association between Black ethnic density and an important mental health outcome, and to identify differential effects of this association across social, economic, and demographic subpopulations. We analyzed the African American sample (n = 3570) from the National Survey of American Life, which we geocoded to the 2000 US Census. We determined the threshold with a multivariable regression spline model. We examined differential effects of ethnic density with random-effects multilevel linear regressions stratified by sociodemographic characteristics. The protective association between Black ethnic density and depressive symptoms changed direction, becoming a detrimental effect, when ethnic density reached 85%. Black ethnic density was protective for lower socioeconomic positions and detrimental for the better-off categories. The masking effects of area deprivation were stronger in the highest levels of Black ethnic density. Addressing racism, racial discrimination, economic deprivation, and poor services-the main drivers differentiating ethnic density from residential segregation-will help to ensure that the racial/ethnic composition of a neighborhood is not a risk factor for poor mental health.